path: root/lib/sqlalchemy/orm/mapper.py

# orm/mapper.py
# Copyright (C) 2005,2006 Michael Bayer mike_mp@zzzcomputing.com
#
# This module is part of SQLAlchemy and is released under
# the MIT License: http://www.opensource.org/licenses/mit-license.php

from sqlalchemy import sql, schema, util, exceptions, logging
from sqlalchemy import sql_util as sqlutil
import util as mapperutil
import sync
from interfaces import MapperProperty, MapperOption, OperationContext
import query as querylib
import session as sessionlib
import weakref

__all__ = ['Mapper', 'MapperExtension', 'class_mapper', 'object_mapper', 'EXT_PASS']

# a dictionary mapping classes to their primary mappers
mapper_registry = weakref.WeakKeyDictionary()

# a list of MapperExtensions that will be installed in all mappers by default
global_extensions = []

# a constant returned by get_attr_by_column to indicate
# this mapper is not handling an attribute for a particular
# column
NO_ATTRIBUTE = object()

# returned by a MapperExtension method to indicate a "do nothing" response
EXT_PASS = object()
                
class Mapper(object):
    """Defines the correlation of class attributes to database table columns.
    
    Instances of this class should be constructed via the sqlalchemy.orm.mapper() function."""
    def __init__(self, 
                class_, 
                local_table, 
                properties = None, 
                primary_key = None, 
                non_primary = False,
                inherits = None, 
                inherit_condition = None, 
                extension = None,
                order_by = False,
                allow_column_override = False,
                entity_name = None,
                always_refresh = False,
                version_id_col = None,
                polymorphic_on=None,
                _polymorphic_map=None,
                polymorphic_identity=None,
                concrete=False,
                select_table=None,
                allow_null_pks=False,
                batch=True,
                column_prefix=None):
        """construct a new mapper.  
        
        All arguments may be sent to the sqlalchemy.orm.mapper() function where they are
        passed through to here.
        
        class_ - the class to be mapped.
        
        local_table - the table to which the class is mapped, or None if this mapper inherits
        from another mapper using concrete table inheritance.
        
        properties - a dictionary mapping the string names of object attributes to MapperProperty 
        instances, which define the persistence behavior of that attribute.  Note that the columns in the 
        mapped table are automatically converted into ColumnProperty instances based on the "key" 
        property of each Column (although they can be overridden using this dictionary).
        
        primary_key - a list of Column objects which define the "primary key" to be used against this mapper's
        selectable unit.  This is normally simply the primary key of the "local_table", but can be overridden here.
        
        non_primary - construct a Mapper that will define only the selection of instances, not their persistence.
        
        inherits - another Mapper for which this Mapper will have an inheritance relationship with.
        
        inherit_condition - for joined table inheritance, a SQL expression (constructed ClauseElement) which 
        will define how the two tables are joined; defaults to a natural join between the two tables.
        
        extension - a MapperExtension instance or list of MapperExtension instances which will be applied to
        all operations by this Mapper.
        
        order_by - a single Column or list of Columns for which selection operations should use as the default
        ordering for entities.  Defaults to the OID/ROWID of the table if any, or the first primary key column of the table.
        
        allow_column_override - if True, allows association relationships to be set up which override the usage of 
        a column that is on the table (based on key/attribute name).
        
        entity_name - a name to be associated with the class, to allow alternate mappings for a single class.
        
        always_refresh - if True, all query operations for this mapped class will overwrite all data
        within object instances that already exist within the session, erasing any in-memory changes with whatever
        information was loaded from the database.
        
        version_id_col - a Column which must have an integer type that will be used to keep a running "version id" of
        mapped entities in the database.  this is used during save operations to insure that no other thread or process
        has updated the instance during the lifetime of the entity, else a ConcurrentModificationError exception is thrown.
        
        polymorphic_on - used with mappers in an inheritance relationship, a Column which will identify the class/mapper
        combination to be used with a particular row.  requires the polymorphic_identity value to be set for all mappers
        in the inheritance hierarchy.
        
        _polymorphic_map - used internally to propigate the full map of polymorphic identifiers to surrogate mappers.
        
        polymorphic_identity - a value which will be stored in the Column denoted by polymorphic_on, corresponding to the
        "class identity" of this mapper.
        
        concrete - if True, indicates this mapper should use concrete table inheritance with its parent mapper.
        
        select_table - a Table or (more commonly) Selectable which will be used to select instances of this mapper's class.
        usually used to provide polymorphic loading among several classes in an inheritance hierarchy.
        
        allow_null_pks - indicates that composite primary keys where one or more (but not all) columns contain NULL is a valid
        primary key.  Primary keys which contain NULL values usually indicate that a result row does not contain an entity
        and should be skipped.
        
        batch - indicates that save operations of multiple entities can be batched together for efficiency.  
        setting to False indicates that an instance will be fully saved before saving the next instance, which 
        includes inserting/updating all table rows corresponding to the entity as well as calling all MapperExtension 
        methods corresponding to the save operation.

        column_prefix - a string which will be prepended to the "key" name of all Columns when creating column-based
        properties from the given Table.  does not affect explicitly specified column-based properties
        """
        if not issubclass(class_, object):
            raise exceptions.ArgumentError("Class '%s' is not a new-style class" % class_.__name__)

        for table in (local_table, select_table):
            if table is not None and isinstance(table, sql._SelectBaseMixin):
                # some db's, noteably postgres, dont want to select from a select
                # without an alias.  also if we make our own alias internally, then
                # the configured properties on the mapper are not matched against the alias 
                # we make, theres workarounds but it starts to get really crazy (its crazy enough
                # the SQL that gets generated) so just require an alias
                raise exceptions.ArgumentError("Mapping against a Select object requires that it has a name.  Use an alias to give it a name, i.e. s = select(...).alias('myselect')")

        self.class_ = class_
        self.entity_name = entity_name
        self.class_key = ClassKey(class_, entity_name)
        self.primary_key = primary_key
        self.non_primary = non_primary
        self.order_by = order_by
        self.always_refresh = always_refresh
        self.version_id_col = version_id_col
        self.concrete = concrete
        self.single = False
        self.inherits = inherits
        self.select_table = select_table
        self.local_table = local_table
        self.inherit_condition = inherit_condition
        self.extension = extension
        self.properties = properties or {}
        self.allow_column_override = allow_column_override
        self.allow_null_pks = allow_null_pks
        self.delete_orphans = []
        self.batch = batch
        self.column_prefix = column_prefix
        # a Column which is used during a select operation to retrieve the 
        # "polymorphic identity" of the row, which indicates which Mapper should be used
        # to construct a new object instance from that row.
        self.polymorphic_on = polymorphic_on
        
        # our 'polymorphic identity', a string name that when located in a result set row
        # indicates this Mapper should be used to construct the object instance for that row.
        self.polymorphic_identity = polymorphic_identity
        
        # a dictionary of 'polymorphic identity' names, associating those names with
        # Mappers that will be used to construct object instances upon a select operation.
        if _polymorphic_map is None:
            self.polymorphic_map = {}
        else:
            self.polymorphic_map = _polymorphic_map

        class LOrderedProp(util.OrderedProperties):
            """this extends OrderedProperties to trigger a compile() before the
            members of the object are accessed."""
            def _get_data(s):
                self.compile()
                return s.__dict__['_OrderedProperties__data']
            _OrderedProperties__data = property(_get_data)
                
        self.columns = LOrderedProp()
        self.c = self.columns
                
        # each time the options() method is called, the resulting Mapper is
        # stored in this dictionary based on the given options for fast re-access
        self._options = {}
        
        # a set of all mappers which inherit from this one.
        self._inheriting_mappers = util.Set()
        
        # a second mapper that is used for selecting, if the "select_table" argument
        # was sent to this mapper.
        self.__surrogate_mapper = None
        
        # whether or not our compile() method has been called already.
        self.__is_compiled = False

        # if this mapper is to be a primary mapper (i.e. the non_primary flag is not set),
        # associate this Mapper with the given class_ and entity name.  subsequent
        # calls to class_mapper() for the class_/entity name combination will return this 
        # mapper.
        self._compile_class()

        self.__log("constructed")
        
        # uncomment to compile at construction time (the old way)
        # this will break mapper setups that arent declared in the order
        # of dependency
        #self.compile()
    
    def __log(self, msg):
        self.logger.info("(" + self.class_.__name__ + "|" + (self.entity_name is not None and "/%s" % self.entity_name or "") + (self.local_table and self.local_table.name or str(self.local_table)) + (not self._is_primary_mapper() and "|non-primary" or "") + ") " + msg)
    
    def __log_debug(self, msg):
        self.logger.debug("(" + self.class_.__name__ + "|" + (self.entity_name is not None and "/%s" % self.entity_name or "") + (self.local_table and self.local_table.name or str(self.local_table)) + (not self._is_primary_mapper() and "|non-primary" or "") + ") " + msg)
            
    def _is_orphan(self, obj):
        optimistic = has_identity(obj)
        for (key,klass) in self.delete_orphans:
            if not getattr(klass, key).hasparent(obj, optimistic=optimistic):
                if not has_identity(obj):
                    raise exceptions.FlushError("instance %s is an unsaved, pending instance and is an orphan (is not attached to any parent '%s' instance via that classes' '%s' attribute)" % (obj, klass.__name__, key))
                return True
        else:
            return False
            
    def _get_props(self):
        self.compile()
        return self.__props
    props = property(_get_props, doc="compiles this mapper if needed, and returns the \
    dictionary of MapperProperty objects associated with this mapper.")
        
    def compile(self):
        """compile this mapper into its final internal format.
        
        this is the 'external' version of the method which is not reentrant."""
        if self.__is_compiled:
            return self
        
        self._compile_all()
        
        # if we're not primary, compile us
        if self.non_primary:
            self._do_compile()
            self._initialize_properties()
                
        return self
    
    def _compile_all(self):
        # compile all primary mappers
        for mapper in mapper_registry.values():
            if not mapper.__is_compiled:
                mapper._do_compile()
                
        # initialize properties on all mappers
        for mapper in mapper_registry.values():
            if not mapper.__props_init:
                mapper._initialize_properties()
        
    def _check_compile(self):
        if self.non_primary:
            self._do_compile()
            self._initialize_properties()
        return self
        
    def _do_compile(self):
        """compile this mapper into its final internal format.  
        
        this is the 'internal' version of the method which is assumed to be called within compile()
        and is reentrant.
        """
        if self.__is_compiled:
            return self
        self.__log("_do_compile() started")
        self.__is_compiled = True
        self.__props_init = False
        self._compile_extensions()
        self._compile_inheritance()
        self._compile_tables()
        self._compile_properties()
        self._compile_selectable()
        self.__log("_do_compile() complete")
        return self
        
    def _compile_extensions(self):
        """goes through the global_extensions list as well as the list of MapperExtensions
        specified for this Mapper and creates a linked list of those extensions."""
        extlist = util.Set()
        for ext_class in global_extensions:
            if isinstance(ext_class, MapperExtension):
                extlist.add(ext_class)
            else:
                extlist.add(ext_class())

        extension = self.extension
        if extension is not None:
            for ext_obj in util.to_list(extension):
                extlist.add(ext_obj)

        self.extension = _ExtensionCarrier()
        for ext in extlist:
            self.extension.append(ext)
        
    def _compile_inheritance(self):
        """determines if this Mapper inherits from another mapper, and if so calculates the mapped_table
        for this Mapper taking the inherited mapper into account.  for joined table inheritance, creates
        a SyncRule that will synchronize column values between the joined tables. also initializes polymorphic variables
        used in polymorphic loads."""
        if self.inherits is not None:
            if isinstance(self.inherits, type):
                self.inherits = class_mapper(self.inherits, compile=False)._do_compile()
            else:
                self.inherits = self.inherits._do_compile()
            if not issubclass(self.class_, self.inherits.class_):
                raise exceptions.ArgumentError("Class '%s' does not inherit from '%s'" % (self.class_.__name__, self.inherits.class_.__name__))
            # inherit_condition is optional.
            if self.local_table is None:
                self.local_table = self.inherits.local_table
                self.single = True
            if not self.local_table is self.inherits.local_table:
                if self.concrete:
                    self._synchronizer= None
                    self.mapped_table = self.local_table
                else:
                    if self.inherit_condition is None:
                        # figure out inherit condition from our table to the immediate table
                        # of the inherited mapper, not its full table which could pull in other 
                        # stuff we dont want (allows test/inheritance.InheritTest4 to pass)
                        self.inherit_condition = sql.join(self.inherits.local_table, self.local_table).onclause
                    self.mapped_table = sql.join(self.inherits.mapped_table, self.local_table, self.inherit_condition)
                    # generate sync rules.  similarly to creating the on clause, specify a 
                    # stricter set of tables to create "sync rules" by,based on the immediate
                    # inherited table, rather than all inherited tables
                    self._synchronizer = sync.ClauseSynchronizer(self, self, sync.ONETOMANY)
                    self._synchronizer.compile(self.mapped_table.onclause)
            else:
                self._synchronizer = None
                self.mapped_table = self.local_table
            if self.polymorphic_identity is not None:
                self.inherits._add_polymorphic_mapping(self.polymorphic_identity, self)
            if self.polymorphic_on is None and self.inherits.polymorphic_on is not None:
                self.polymorphic_on = self.mapped_table.corresponding_column(self.inherits.polymorphic_on, keys_ok=True, raiseerr=False)
            if self.order_by is False:
                self.order_by = self.inherits.order_by
            self.polymorphic_map = self.inherits.polymorphic_map
            self.batch = self.inherits.batch
            self.inherits._inheriting_mappers.add(self)
        else:
            self._synchronizer = None
            self.mapped_table = self.local_table
            if self.polymorphic_identity is not None:
                self._add_polymorphic_mapping(self.polymorphic_identity, self)

        if self.mapped_table is None:
            raise exceptions.ArgumentError("Mapper '%s' does not have a mapped_table specified.  (Are you using the return value of table.create()?  It no longer has a return value.)" % str(self))
            
        # convert polymorphic class associations to mappers
        for key in self.polymorphic_map.keys():
            if isinstance(self.polymorphic_map[key], type):
                self.polymorphic_map[key] = class_mapper(self.polymorphic_map[key])

    def _add_polymorphic_mapping(self, key, class_or_mapper, entity_name=None):
        """adds a Mapper to our 'polymorphic map' """
        if isinstance(class_or_mapper, type):
            class_or_mapper = class_mapper(class_or_mapper, entity_name=entity_name)
        self.polymorphic_map[key] = class_or_mapper

    def _compile_tables(self):
        """after the inheritance relationships have been reconciled, sets up some more table-based instance
        variables and determines the "primary key" columns for all tables represented by this Mapper."""

        # summary of the various Selectable units:
        # mapped_table - the Selectable that represents a join of the underlying Tables to be saved (or just the Table)
        # local_table - the Selectable that was passed to this Mapper's constructor, if any
        # select_table - the Selectable that will be used during queries.  if this is specified
        # as a constructor keyword argument, it takes precendence over mapped_table, otherwise its mapped_table
        # unjoined_table - our Selectable, minus any joins constructed against the inherits table.
        # this is either select_table if it was given explicitly, or in the case of a mapper that inherits
        # its local_table
        # tables - a collection of underlying Table objects pulled from mapped_table

        if self.select_table is None:
            self.select_table = self.mapped_table
        self.unjoined_table = self.local_table

        # locate all tables contained within the "table" passed in, which
        # may be a join or other construct
        self.tables = sqlutil.TableFinder(self.mapped_table)

        # determine primary key columns, either passed in, or get them from our set of tables
        self.pks_by_table = {}
        if self.primary_key is not None:
            # determine primary keys using user-given list of primary key columns as a guide
            #
            # TODO: this might not work very well for joined-table and/or polymorphic 
            # inheritance mappers since local_table isnt taken into account nor is select_table
            # need to test custom primary key columns used with inheriting mappers
            for k in self.primary_key:
                self.pks_by_table.setdefault(k.table, util.OrderedSet()).add(k)
                if k.table != self.mapped_table:
                    # associate pk cols from subtables to the "main" table
                    self.pks_by_table.setdefault(self.mapped_table, util.OrderedSet()).add(k)
        else:
            # no user-defined primary key columns - go through all of our represented tables
            # and assemble primary key columns
            for t in self.tables + [self.mapped_table]:
                try:
                    l = self.pks_by_table[t]
                except KeyError:
                    l = self.pks_by_table.setdefault(t, util.OrderedSet())
                for k in t.primary_key:
                    #if k.key not in t.c and k._label not in t.c:
                        # this is a condition that was occurring when table reflection was doubling up primary keys
                        # that were overridden in the Table constructor
                    #    raise exceptions.AssertionError("Column " + str(k) + " not located in the column set of table " + str(t))
                    l.add(k)

        if len(self.pks_by_table[self.mapped_table]) == 0:
            raise exceptions.ArgumentError("Could not assemble any primary key columns for mapped table '%s'" % (self.mapped_table.name))


    def _compile_properties(self):
        """inspects the properties dictionary sent to the Mapper's constructor as well as the mapped_table, and creates
        MapperProperty objects corresponding to each mapped column and relation.  also grabs MapperProperties from the 
        inherited mapper, if any, and creates copies of them to attach to this Mapper."""
        # object attribute names mapped to MapperProperty objects
        self.__props = {}

        # table columns mapped to lists of MapperProperty objects
        # using a list allows a single column to be defined as 
        # populating multiple object attributes
        self.columntoproperty = mapperutil.TranslatingDict(self.mapped_table)

        # load custom properties 
        if self.properties is not None:
            for key, prop in self.properties.iteritems():
                self._compile_property(key, prop, False)

        if self.inherits is not None:
            for key, prop in self.inherits.__props.iteritems():
                if not self.__props.has_key(key):
                    prop.adapt_to_inherited(key, self)

        # load properties from the main table object,
        # not overriding those set up in the 'properties' argument
        for column in self.mapped_table.columns:
            if self.columntoproperty.has_key(column):
                continue
            if not self.columns.has_key(column.key):
                self.columns[column.key] = self.select_table.corresponding_column(column, keys_ok=True, raiseerr=True)

            column_key = (self.column_prefix or '') + column.key
            prop = self.__props.get(column.key, None)
            if prop is None:
                prop = ColumnProperty(column)
                self.__props[column_key] = prop
                prop.set_parent(self)
                self.__log("adding ColumnProperty %s" % (column_key))
            elif isinstance(prop, ColumnProperty):
                if prop.parent is not self:
                    prop = ColumnProperty(deferred=prop.deferred, group=prop.group, *prop.columns)
                    prop.set_parent(self)
                    self.__props[column_key] = prop
                prop.columns.append(column)
                self.__log("appending to existing ColumnProperty %s" % (column_key))
            else:
                if not self.allow_column_override:
                    raise exceptions.ArgumentError("WARNING: column '%s' not being added due to property '%s'.  Specify 'allow_column_override=True' to mapper() to ignore this condition." % (column.key, repr(prop)))
                else:
                    continue

            # its a ColumnProperty - match the ultimate table columns
            # back to the property
            proplist = self.columntoproperty.setdefault(column, [])
            proplist.append(prop)


    def _initialize_properties(self):
        """calls the init() method on all MapperProperties attached to this mapper.  this will incur the
        compilation of related mappers."""
        self.__log("_initialize_properties() started")
        l = [(key, prop) for key, prop in self.__props.iteritems()]
        for key, prop in l:
            if getattr(prop, 'key', None) is None:
                prop.init(key, self)
        self.__log("_initialize_properties() complete")
        self.__props_init = True
        
    def _compile_selectable(self):
        """if the 'select_table' keyword argument was specified, 
        set up a second "surrogate mapper" that will be used for select operations.
        the columns of select_table should encompass all the columns of the mapped_table either directly
        or through proxying relationships."""
        if self.select_table is not self.mapped_table:
            props = {}
            if self.properties is not None:
                for key, prop in self.properties.iteritems():
                    if sql.is_column(prop):
                        props[key] = self.select_table.corresponding_column(prop)
                    elif (isinstance(prop, list) and sql.is_column(prop[0])):
                        props[key] = [self.select_table.corresponding_column(c) for c in prop]
            self.__surrogate_mapper = Mapper(self.class_, self.select_table, non_primary=True, properties=props, _polymorphic_map=self.polymorphic_map, polymorphic_on=self.select_table.corresponding_column(self.polymorphic_on))

    def _compile_class(self):
        """if this mapper is to be a primary mapper (i.e. the non_primary flag is not set),
        associate this Mapper with the given class_ and entity name.  subsequent
        calls to class_mapper() for the class_/entity name combination will return this 
        mapper.  also decorates the __init__ method on the mapped class to include auto-session attachment logic."""
        if self.non_primary:
            return
        
        if not self.non_primary and (mapper_registry.has_key(self.class_key)):
             raise exceptions.ArgumentError("Class '%s' already has a primary mapper defined with entity name '%s'.  Use non_primary=True to create a non primary Mapper, or to create a new primary mapper, remove this mapper first via sqlalchemy.orm.clear_mapper(mapper), or preferably sqlalchemy.orm.clear_mappers() to clear all mappers." % (self.class_, self.entity_name))

        sessionlib.attribute_manager.reset_class_managed(self.class_)
    
        oldinit = self.class_.__init__
        def init(self, *args, **kwargs):
            entity_name = kwargs.pop('_sa_entity_name', None)
            mapper = mapper_registry.get(ClassKey(self.__class__, entity_name))
            if mapper is not None:
                mapper = mapper.compile()

                # this gets the AttributeManager to do some pre-initialization,
                # in order to save on KeyErrors later on
                sessionlib.attribute_manager.init_attr(self)

            if kwargs.has_key('_sa_session'):
                session = kwargs.pop('_sa_session')
            else:
                # works for whatever mapper the class is associated with
                if mapper is not None:
                    session = mapper.extension.get_session()
                    if session is EXT_PASS:
                        session = None
                else:
                    session = None
            # if a session was found, either via _sa_session or via mapper extension,
            # and we have found a mapper, save() this instance to the session, and give it an associated entity_name.
            # otherwise, this instance will not have a session or mapper association until it is
            # save()d to some session.
            if session is not None and mapper is not None:
                self._entity_name = entity_name
                session._register_pending(self)
                
            if oldinit is not None:
                try:
                    oldinit(self, *args, **kwargs)
                except:
                    if session is not None:
                        session.expunge(self)
                    raise
        # override oldinit, insuring that its not already a Mapper-decorated init method
        if oldinit is None or not hasattr(oldinit, '_sa_mapper_init'):
            init._sa_mapper_init = True
            try:
                init.__name__ = oldinit.__name__
                init.__doc__ = oldinit.__doc__
            except:
                # cant set __name__ in py 2.3 !
                pass
            self.class_.__init__ = init
        mapper_registry[self.class_key] = self
        if self.entity_name is None:
            self.class_.c = self.c
            
    def base_mapper(self):
        """return the ultimate base mapper in an inheritance chain"""
        if self.inherits is not None:
            return self.inherits.base_mapper()
        else:
            return self
    
    def common_parent(self, other):
        """return true if the given mapper shares a common inherited parent as this mapper"""
        return self.base_mapper() is other.base_mapper()
        
    def isa(self, other):
        """return True if the given mapper inherits from this mapper"""
        m = other
        while m is not self and m.inherits is not None:
            m = m.inherits
        return m is self

    def iterate_to_root(self):
        m = self
        while m is not None:
            yield m
            m = m.inherits
    
    def polymorphic_iterator(self):
        m = self.base_mapper()
        def iterate(m):
            yield m
            for mapper in m._inheriting_mappers:
                for x in iterate(mapper):
                    yield x
        return iterate(m)
                
    def add_properties(self, dict_of_properties):
        """adds the given dictionary of properties to this mapper, using add_property."""
        for key, value in dict_of_properties.iteritems():
            self.add_property(key, value)

    def add_property(self, key, prop):
        """add an indiviual MapperProperty to this mapper.  
        
        If the mapper has not been compiled yet, just adds the property to the initial 
        properties dictionary sent to the constructor.  if this Mapper
        has already been compiled, then the given MapperProperty is compiled immediately."""
        self.properties[key] = prop
        if self.__is_compiled:
            # if we're compiled, make sure all the other mappers are compiled too
            self._compile_all()
            self._compile_property(key, prop, init=True)
            
    def _create_prop_from_column(self, column, skipmissing=False):
        if sql.is_column(column):
            try:
                column = self.mapped_table.corresponding_column(column)
            except KeyError:
                if skipmissing:
                    return
                raise exceptions.ArgumentError("Column '%s' is not represented in mapper's table" % prop._label)
            return ColumnProperty(column)
        elif isinstance(column, list) and sql.is_column(column[0]):
            try:
                column = [self.mapped_table.corresponding_column(c) for c in column]
            except KeyError, e:
                # TODO: want to take the columns we have from this
                if skipmissing:
                    return
                raise exceptions.ArgumentError("Column '%s' is not represented in mapper's table" % e.args[0])
            return ColumnProperty(*column)
        else:
            return None

    def _compile_property(self, key, prop, init=True, skipmissing=False, setparent=True):
        """add a MapperProperty to this or another Mapper, including configuration of the property.
        
        The properties' parent attribute will be set, and the property will also be 
        copied amongst the mappers which inherit from this one.
        
        if the given prop is a Column or list of Columns, a ColumnProperty will be created.
        """
        self.__log("_compile_property(%s, %s)" % (key, prop.__class__.__name__))

        if not isinstance(prop, MapperProperty):
            prop = self._create_prop_from_column(prop, skipmissing=skipmissing)
            if prop is None:
                raise exceptions.ArgumentError("'%s' is not an instance of MapperProperty or Column" % repr(prop))

        self.__props[key] = prop
        if setparent:
            prop.set_parent(self)
            
        if isinstance(prop, ColumnProperty):
            col = self.select_table.corresponding_column(prop.columns[0], keys_ok=True, raiseerr=False)
            if col is None:
                col = prop.columns[0]
            self.columns[key] = col
            for col in prop.columns:
                proplist = self.columntoproperty.setdefault(col, [])
                proplist.append(prop)

        if init:
            prop.init(key, self)

        for mapper in self._inheriting_mappers:
            prop.adapt_to_inherited(key, mapper)

    def __str__(self):
        return "Mapper|" + self.class_.__name__ + "|" + (self.entity_name is not None and "/%s" % self.entity_name or "") + (self.local_table and self.local_table.name or str(self.local_table)) + (not self._is_primary_mapper() and "|non-primary" or "")
    
    def _is_primary_mapper(self):
        """returns True if this mapper is the primary mapper for its class key (class + entity_name)"""
        return mapper_registry.get(self.class_key, None) is self

    def primary_mapper(self):
        """returns the primary mapper corresponding to this mapper's class key (class + entity_name)"""
        return mapper_registry[self.class_key]

    def is_assigned(self, instance):
        """return True if this mapper handles the given instance.  
        
        this is dependent not only on class assignment but the optional "entity_name" parameter as well."""
        return instance.__class__ is self.class_ and getattr(instance, '_entity_name', None) == self.entity_name

    def _assign_entity_name(self, instance):
        """assign this Mapper's entity name to the given instance.  
        
        subsequent Mapper lookups for this instance will return the primary 
        mapper corresponding to this Mapper's class and entity name."""
        instance._entity_name = self.entity_name
        
    def get_session(self):
        """return the contextual session provided by the mapper extension chain, if any.
        
        raises InvalidRequestError if a session cannot be retrieved from the extension chain
        """
        self.compile()
        s = self.extension.get_session()
        if s is EXT_PASS:
            raise exceptions.InvalidRequestError("No contextual Session is established.  Use a MapperExtension that implements get_session or use 'import sqlalchemy.mods.threadlocal' to establish a default thread-local contextual session.")
        return s
    
    def has_eager(self):
        """return True if one of the properties attached to this Mapper is eager loading"""
        return getattr(self, '_has_eager', False)
    
    def instances(self, cursor, session, *mappers, **kwargs):
        """return a list of mapped instances corresponding to the rows in a given ResultProxy."""
        return querylib.Query(self, session).instances(cursor, *mappers, **kwargs)

    def identity_key_from_row(self, row):
        """return an identity-map key for use in storing/retrieving an item from the identity map.

        row - a sqlalchemy.dbengine.RowProxy instance or other map corresponding result-set
        column names to their values within a row.
        """
        return (self.class_, tuple([row[column] for column in self.pks_by_table[self.mapped_table]]), self.entity_name)
        
    def identity_key_from_primary_key(self, primary_key):
        """return an identity-map key for use in storing/retrieving an item from an identity map.
        
        primary_key - a list of values indicating the identifier.
        """
        return (self.class_, tuple(util.to_list(primary_key)), self.entity_name)

    def identity_key_from_instance(self, instance):
        """return the identity key for the given instance, based on its primary key attributes.
        
        this value is typically also found on the instance itself under the attribute name '_instance_key'.
        """
        return self.identity_key_from_primary_key(self.primary_key_from_instance(instance))

    def primary_key_from_instance(self, instance):
        """return the list of primary key values for the given instance."""
        return [self.get_attr_by_column(instance, column) for column in self.pks_by_table[self.mapped_table]]

    def instance_key(self, instance):
        """deprecated.  a synonym for identity_key_from_instance."""
        return self.identity_key_from_instance(instance)

    def identity_key(self, primary_key):
        """deprecated.  a synonym for identity_key_from_primary_key."""
        return self.identity_key_from_primary_key(primary_key)

    def identity(self, instance):
        """deprecated. a synoynm for primary_key_from_instance."""
        return self.primary_key_from_instance(instance)

    def _getpropbycolumn(self, column, raiseerror=True):
        try:
            prop = self.columntoproperty[column]
        except KeyError:
            try:
                prop = self.__props[column.key]
                if not raiseerror:
                    return None
                raise exceptions.InvalidRequestError("Column '%s.%s' is not available, due to conflicting property '%s':%s" % (column.table.name, column.name, column.key, repr(prop)))
            except KeyError:
                if not raiseerror:
                    return None
                raise exceptions.InvalidRequestError("No column %s.%s is configured on mapper %s..." % (column.table.name, column.name, str(self)))
        return prop[0]
        
    def get_attr_by_column(self, obj, column, raiseerror=True):
        """return an instance attribute using a Column as the key."""
        prop = self._getpropbycolumn(column, raiseerror)
        if prop is None:
            return NO_ATTRIBUTE
        #self.__log_debug("get column attribute '%s' from instance %s" % (column.key, mapperutil.instance_str(obj)))
        return prop.getattr(obj)

    def set_attr_by_column(self, obj, column, value):
        """set the value of an instance attribute using a Column as the key."""
        self.columntoproperty[column][0].setattr(obj, value)
    
    def save_obj(self, objects, uowtransaction, postupdate=False, post_update_cols=None, single=False):
        """issue INSERT and/or UPDATE statements for a list of objects.

        this is called within the context of a UOWTransaction during a flush operation.
        
        save_obj issues SQL statements not just for instances mapped directly by this mapper, but
        for instances mapped by all inheriting mappers as well.  This is to maintain proper insert
        ordering among a polymorphic chain of instances. Therefore save_obj is typically 
        called only on a "base mapper", or a mapper which does not inherit from any other mapper."""
        
        self.__log_debug("save_obj() start, " + (single and "non-batched" or "batched"))
        
        # if batch=false, call save_obj separately for each object
        if not single and not self.batch:
            for obj in objects:
                self.save_obj([obj], uowtransaction, postupdate=postupdate, post_update_cols=post_update_cols, single=True)
            return
            
        connection = uowtransaction.transaction.connection(self)

        if not postupdate:
            for obj in objects:
                if not has_identity(obj):
                    for mapper in object_mapper(obj).iterate_to_root():
                        mapper.extension.before_insert(mapper, connection, obj)
                else:
                    for mapper in object_mapper(obj).iterate_to_root():
                        mapper.extension.before_update(mapper, connection, obj)

        for obj in objects:
            # detect if we have a "pending" instance (i.e. has no instance_key attached to it),
            # and another instance with the same identity key already exists as persistent.  convert to an 
            # UPDATE if so.
            mapper = object_mapper(obj)
            instance_key = mapper.instance_key(obj)
            is_row_switch = not postupdate and not has_identity(obj) and instance_key in uowtransaction.uow.identity_map
            if is_row_switch:
                existing = uowtransaction.uow.identity_map[instance_key]
                if not uowtransaction.is_deleted(existing):
                    raise exceptions.FlushError("New instance %s with identity key %s conflicts with persistent instance %s" % (mapperutil.instance_str(obj), str(instance_key), mapperutil.instance_str(existing)))
                self.__log_debug("detected row switch for identity %s.  will update %s, remove %s from transaction" % (instance_key, mapperutil.instance_str(obj), mapperutil.instance_str(existing)))
                uowtransaction.unregister_object(existing)
            
        inserted_objects = util.Set()
        updated_objects = util.Set()
        
        table_to_mapper = {}
        for mapper in self.polymorphic_iterator():
            for t in mapper.tables:
                table_to_mapper.setdefault(t, mapper)

        for table in sqlutil.TableCollection(list(table_to_mapper.keys())).sort(reverse=False):
            # two lists to store parameters for each table/object pair located
            insert = []
            update = []
            
            for obj in objects:
                mapper = object_mapper(obj)
                if table not in mapper.tables or not mapper._has_pks(table):
                    continue
                instance_key = mapper.instance_key(obj)
                self.__log_debug("save_obj() table '%s' instance %s identity %s" % (table.name, mapperutil.instance_str(obj), str(instance_key)))

                isinsert = not instance_key in uowtransaction.uow.identity_map and not postupdate and not has_identity(obj)
                params = {}
                hasdata = False
                for col in table.columns:
                    if col is mapper.version_id_col:
                        if not isinsert:
                            params[col._label] = mapper.get_attr_by_column(obj, col)
                            params[col.key] = params[col._label] + 1
                        else:
                            params[col.key] = 1
                    elif col in mapper.pks_by_table[table]:
                        # column is a primary key ?
                        if not isinsert:
                            # doing an UPDATE?  put primary key values as "WHERE" parameters
                            # matching the bindparam we are creating below, i.e. "<tablename>_<colname>"
                            params[col._label] = mapper.get_attr_by_column(obj, col)
                        else:
                            # doing an INSERT, primary key col ? 
                            # if the primary key values are not populated,
                            # leave them out of the INSERT altogether, since PostGres doesn't want
                            # them to be present for SERIAL to take effect.  A SQLEngine that uses
                            # explicit sequences will put them back in if they are needed
                            value = mapper.get_attr_by_column(obj, col)
                            if value is not None:
                                params[col.key] = value
                    elif mapper.polymorphic_on is not None and mapper.polymorphic_on.shares_lineage(col):
                        if isinsert:
                            self.__log_debug("Using polymorphic identity '%s' for insert column '%s'" % (mapper.polymorphic_identity, col.key))
                            value = mapper.polymorphic_identity
                            if col.default is None or value is not None:
                                params[col.key] = value
                    else:
                        # column is not a primary key ?
                        if not isinsert:
                            # doing an UPDATE ? get the history for the attribute, with "passive"
                            # so as not to trigger any deferred loads.  if there is a new
                            # value, add it to the bind parameters
                            if post_update_cols is not None and col not in post_update_cols:
                                continue
                            elif is_row_switch:
                                params[col.key] = self.get_attr_by_column(obj, col)
                                hasdata = True
                                continue
                            prop = mapper._getpropbycolumn(col, False)
                            if prop is None:
                                continue
                            history = prop.get_history(obj, passive=True)
                            if history:
                                a = history.added_items()
                                if len(a):
                                    params[col.key] = a[0]
                                    hasdata = True
                        else:
                            # doing an INSERT, non primary key col ? 
                            # add the attribute's value to the 
                            # bind parameters, unless its None and the column has a 
                            # default.  if its None and theres no default, we still might
                            # not want to put it in the col list but SQLIte doesnt seem to like that
                            # if theres no columns at all
                            value = mapper.get_attr_by_column(obj, col, False)
                            if value is NO_ATTRIBUTE:
                                continue
                            if col.default is None or value is not None:
                                params[col.key] = value

                if not isinsert:
                    if hasdata:
                        # if none of the attributes changed, dont even
                        # add the row to be updated.
                        update.append((obj, params, mapper))
                else:
                    insert.append((obj, params, mapper))

            if len(update):
                mapper = table_to_mapper[table]
                clause = sql.and_()
                for col in mapper.pks_by_table[table]:
                    clause.clauses.append(col == sql.bindparam(col._label, type=col.type))
                if mapper.version_id_col is not None:
                    clause.clauses.append(mapper.version_id_col == sql.bindparam(mapper.version_id_col._label, type=col.type))
                statement = table.update(clause)
                rows = 0
                supports_sane_rowcount = True
                def comparator(a, b):
                    for col in mapper.pks_by_table[table]:
                        x = cmp(a[1][col._label],b[1][col._label])
                        if x != 0:
                            return x
                    return 0
                update.sort(comparator)
                for rec in update:
                    (obj, params, mapper) = rec
                    c = connection.execute(statement, params)
                    mapper._postfetch(connection, table, obj, c, c.last_updated_params())

                    updated_objects.add(obj)
                    rows += c.cursor.rowcount

                if c.supports_sane_rowcount() and rows != len(update):
                    raise exceptions.ConcurrentModificationError("Updated rowcount %d does not match number of objects updated %d" % (rows, len(update)))

            if len(insert):
                statement = table.insert()
                def comparator(a, b):
                    return cmp(a[0]._sa_insert_order, b[0]._sa_insert_order)
                insert.sort(comparator)
                for rec in insert:
                    (obj, params, mapper) = rec
                    c = connection.execute(statement, params)
                    primary_key = c.last_inserted_ids()
                    if primary_key is not None:
                        i = 0
                        for col in mapper.pks_by_table[table]:
                            if mapper.get_attr_by_column(obj, col) is None and len(primary_key) > i:
                                mapper.set_attr_by_column(obj, col, primary_key[i])
                            i+=1
                    mapper._postfetch(connection, table, obj, c, c.last_inserted_params())
                    
                    # synchronize newly inserted ids from one table to the next
                    # TODO: this fires off more than needed, try to organize syncrules
                    # per table
                    def sync(mapper):
                        inherit = mapper.inherits
                        if inherit is not None:
                            sync(inherit)
                        if mapper._synchronizer is not None:
                            mapper._synchronizer.execute(obj, obj)
                    sync(mapper)
                    
                    inserted_objects.add(obj)
        if not postupdate:
            for obj in inserted_objects:
                for mapper in object_mapper(obj).iterate_to_root():
                    mapper.extension.after_insert(mapper, connection, obj)
            for obj in updated_objects:
                for mapper in object_mapper(obj).iterate_to_root():
                    mapper.extension.after_update(mapper, connection, obj)

    def _postfetch(self, connection, table, obj, resultproxy, params):
        """after an INSERT or UPDATE, asks the returned result if PassiveDefaults fired off on the database side
        which need to be post-fetched, *or* if pre-exec defaults like ColumnDefaults were fired off
        and should be populated into the instance. this is only for non-primary key columns."""
        if resultproxy.lastrow_has_defaults():
            clause = sql.and_()
            for p in self.pks_by_table[table]:
                clause.clauses.append(p == self.get_attr_by_column(obj, p))
            row = connection.execute(table.select(clause), None).fetchone()
            for c in table.c:
                if self.get_attr_by_column(obj, c, False) is None:
                    self.set_attr_by_column(obj, c, row[c])
        else:
            for c in table.c:
                if c.primary_key or not params.has_key(c.name):
                    continue
                v = self.get_attr_by_column(obj, c, False)
                if v is NO_ATTRIBUTE:
                    continue
                elif v != params.get_original(c.name):
                    self.set_attr_by_column(obj, c, params.get_original(c.name))

    def delete_obj(self, objects, uowtransaction):
        """issue DELETE statements for a list of objects.
        
        this is called within the context of a UOWTransaction during a flush operation."""

        self.__log_debug("delete_obj() start")

        connection = uowtransaction.transaction.connection(self)

        [self.extension.before_delete(self, connection, obj) for obj in objects]
        deleted_objects = util.Set()
        for table in self.tables.sort(reverse=True):
            if not self._has_pks(table):
                continue
            delete = []
            for obj in objects:
                params = {}
                if not hasattr(obj, "_instance_key"):
                    continue
                else:
                    delete.append(params)
                for col in self.pks_by_table[table]:
                    params[col.key] = self.get_attr_by_column(obj, col)
                if self.version_id_col is not None:
                    params[self.version_id_col.key] = self.get_attr_by_column(obj, self.version_id_col)
                deleted_objects.add(obj)
            if len(delete):
                def comparator(a, b):
                    for col in self.pks_by_table[table]:
                        x = cmp(a[col.key],b[col.key])
                        if x != 0:
                            return x
                    return 0
                delete.sort(comparator)
                clause = sql.and_()
                for col in self.pks_by_table[table]:
                    clause.clauses.append(col == sql.bindparam(col.key, type=col.type))
                if self.version_id_col is not None:
                    clause.clauses.append(self.version_id_col == sql.bindparam(self.version_id_col.key, type=self.version_id_col.type))
                statement = table.delete(clause)
                c = connection.execute(statement, delete)
                if c.supports_sane_rowcount() and c.rowcount != len(delete):
                    raise exceptions.ConcurrentModificationError("Updated rowcount %d does not match number of objects updated %d" % (c.cursor.rowcount, len(delete)))
                    
        [self.extension.after_delete(self, connection, obj) for obj in deleted_objects]

    def _has_pks(self, table):
        try:
            for k in self.pks_by_table[table]:
                if not self.columntoproperty.has_key(k):
                    return False
            else:
                return True
        except KeyError:
            return False
            
    def register_dependencies(self, uowcommit, *args, **kwargs):
        """register DependencyProcessor instances with a unitofwork.UOWTransaction.
        
        this calls register_dependencies on all attached MapperProperty instances."""
        for prop in self.__props.values():
            prop.register_dependencies(uowcommit, *args, **kwargs)
    
    def cascade_iterator(self, type, object, recursive=None):
        """iterate each element in an object graph, for all relations taht meet the given cascade rule.
        
        type - the name of the cascade rule (i.e. save-update, delete, etc.)
        
        object - the lead object instance.  child items will be processed per the relations
        defined for this object's mapper.
        
        recursive - used by the function for internal context during recursive calls, leave as None."""
        if recursive is None:
            recursive=util.Set()
        for prop in self.__props.values():
            for c in prop.cascade_iterator(type, object, recursive):
                yield c

    def cascade_callable(self, type, object, callable_, recursive=None):
        """execute a callable for each element in an object graph, for all relations that meet the given cascade rule.
        
        type - the name of the cascade rule (i.e. save-update, delete, etc.)
        
        object - the lead object instance.  child items will be processed per the relations
        defined for this object's mapper.
        
        callable_ - the callable function.
        
        recursive - used by the function for internal context during recursive calls, leave as None."""
        if recursive is None:
            recursive=util.Set()
        for prop in self.__props.values():
            prop.cascade_callable(type, object, callable_, recursive)
            

    def get_select_mapper(self):
        """return the mapper used for issuing selects.
        
        this mapper is the same mapper as 'self' unless the select_table argument was specified for this mapper."""
        return self.__surrogate_mapper or self
        
    def _instance(self, context, row, result = None):
        """pulls an object instance from the given row and appends it to the given result
        list. if the instance already exists in the given identity map, its not added.  in
        either case, executes all the property loaders on the instance to also process extra
        information in the row."""

        if self.polymorphic_on is not None:
            discriminator = row[self.polymorphic_on]
            mapper = self.polymorphic_map[discriminator]
            if mapper is not self:
                row = self.translate_row(mapper, row)
                return mapper._instance(context, row, result=result)
        
        # look in main identity map.  if its there, we dont do anything to it,
        # including modifying any of its related items lists, as its already
        # been exposed to being modified by the application.
        
        populate_existing = context.populate_existing or self.always_refresh
        identitykey = self.identity_key_from_row(row)
        if context.session.has_key(identitykey):
            instance = context.session._get(identitykey)
            self.__log_debug("_instance(): using existing instance %s identity %s" % (mapperutil.instance_str(instance), str(identitykey)))
            isnew = False
            if context.version_check and self.version_id_col is not None and self.get_attr_by_column(instance, self.version_id_col) != row[self.version_id_col]:
                raise exceptions.ConcurrentModificationError("Instance '%s' version of %s does not match %s" % (instance, self.get_attr_by_column(instance, self.version_id_col), row[self.version_id_col]))
                        
            if populate_existing or context.session.is_expired(instance, unexpire=True):
                if not context.identity_map.has_key(identitykey):
                    context.identity_map[identitykey] = instance
                for prop in self.__props.values():
                    prop.execute(context, instance, row, identitykey, True)
            if self.extension.append_result(self, context, row, instance, identitykey, result, isnew) is EXT_PASS:
                if result is not None:
                    result.append(instance)
            return instance
        else:
            self.__log_debug("_instance(): identity key %s not in session" % str(identitykey) + repr([mapperutil.instance_str(x) for x in context.session]))
        # look in result-local identitymap for it.
        exists = context.identity_map.has_key(identitykey)      
        if not exists:
            if self.allow_null_pks:
                # check if *all* primary key cols in the result are None - this indicates 
                # an instance of the object is not present in the row.  
                for col in self.pks_by_table[self.mapped_table]:
                    if row[col] is not None:
                        break
                else:
                    return None
            else:
                # otherwise, check if *any* primary key cols in the result are None - this indicates 
                # an instance of the object is not present in the row.  
                for col in self.pks_by_table[self.mapped_table]:
                    if row[col] is None:
                        return None
            
            # plugin point
            instance = self.extension.create_instance(self, context, row, self.class_)
            if instance is EXT_PASS:
                instance = self._create_instance(context.session)
            self.__log_debug("_instance(): created new instance %s identity %s" % (mapperutil.instance_str(instance), str(identitykey)))
            context.identity_map[identitykey] = instance
            isnew = True
        else:
            instance = context.identity_map[identitykey]
            isnew = False

        # call further mapper properties on the row, to pull further 
        # instances from the row and possibly populate this item.
        if self.extension.populate_instance(self, context, row, instance, identitykey, isnew) is EXT_PASS:
            self.populate_instance(context, instance, row, identitykey, isnew)
        if self.extension.append_result(self, context, row, instance, identitykey, result, isnew) is EXT_PASS:
            if result is not None:
                result.append(instance)
        return instance

    def _create_instance(self, session):
        obj = self.class_.__new__(self.class_)
        obj._entity_name = self.entity_name
        
        # this gets the AttributeManager to do some pre-initialization,
        # in order to save on KeyErrors later on
        sessionlib.attribute_manager.init_attr(obj)

        return obj

    def translate_row(self, tomapper, row):
        """translate the column keys of a row into a new or proxied row that
        can be understood by another mapper.

        This can be used in conjunction with populate_instance to populate
        an instance using an alternate mapper."""
        newrow = util.DictDecorator(row)
        for c in tomapper.mapped_table.c:
            c2 = self.mapped_table.corresponding_column(c, keys_ok=True, raiseerr=True)
            if row.has_key(c2):
                newrow[c] = row[c2]
        return newrow
        
    def populate_instance(self, selectcontext, instance, row, identitykey, isnew):
        """populate an instance from a result row.
        
        This method iterates through the list of MapperProperty objects attached to this Mapper
        and calls each properties execute() method."""
        for prop in self.__props.values():
            prop.execute(selectcontext, instance, row, identitykey, isnew)

Mapper.logger = logging.class_logger(Mapper)


class MapperExtension(object):
    """base implementation for an object that provides overriding behavior to various
    Mapper functions.  For each method in MapperExtension, a result of EXT_PASS indicates
    the functionality is not overridden."""
    def get_session(self):
        """retrieve a contextual Session instance with which to register a new object. 
        
        Note: this is not called if a session is provided with the __init__ params (i.e. _sa_session)"""
        return EXT_PASS
    def load(self, query, *args, **kwargs):
        """override the load method of the Query object.

        the return value of this method is used as the result of query.load() if the
        value is anything other than EXT_PASS."""
        return EXT_PASS
    def get(self, query, *args, **kwargs):
        """override the get method of the Query object.

        the return value of this method is used as the result of query.get() if the
        value is anything other than EXT_PASS."""
        return EXT_PASS
    def get_by(self, query, *args, **kwargs):
        """override the get_by method of the Query object.

        the return value of this method is used as the result of query.get_by() if the
        value is anything other than EXT_PASS."""
        return EXT_PASS
    def select_by(self, query, *args, **kwargs):
        """override the select_by method of the Query object.
        
        the return value of this method is used as the result of query.select_by() if the
        value is anything other than EXT_PASS."""
        return EXT_PASS
    def select(self, query, *args, **kwargs):
        """override the select method of the Query object.
        
        the return value of this method is used as the result of query.select() if the
        value is anything other than EXT_PASS."""
        return EXT_PASS
    def create_instance(self, mapper, selectcontext, row, class_):
        """receieve a row when a new object instance is about to be created from that row.  
        the method can choose to create the instance itself, or it can return 
        None to indicate normal object creation should take place.
        
        mapper - the mapper doing the operation

        selectcontext - SelectionContext corresponding to the instances() call
        
        row - the result row from the database
        
        class_ - the class we are mapping.
        """
        return EXT_PASS
    def append_result(self, mapper, selectcontext, row, instance, identitykey, result, isnew):
        """receive an object instance before that instance is appended to a result list.
        
        If this method returns EXT_PASS, result appending will proceed normally.  
        if this method returns any other value or None, result appending will not proceed for 
        this instance, giving this extension an opportunity to do the appending itself, if desired.

        mapper - the mapper doing the operation
        
        selectcontext - SelectionContext corresponding to the instances() call
        
        row - the result row from the database
        
        instance - the object instance to be appended to the result
        
        identitykey - the identity key of the instance

        result - list to which results are being appended
        
        isnew - indicates if this is the first time we have seen this object instance in the current result
        set.  if you are selecting from a join, such as an eager load, you might see the same object instance
        many times in the same result set.
        """
        return EXT_PASS
    def populate_instance(self, mapper, selectcontext, row, instance, identitykey, isnew):
        """receive a newly-created instance before that instance has its attributes populated.
        
        The normal population of attributes is according to each
        attribute's corresponding MapperProperty (which includes column-based attributes as well
        as relationships to other classes).  If this method returns EXT_PASS, instance population
        will proceed normally.  If any other value or None is returned, instance population
        will not proceed, giving this extension an opportunity to populate the instance itself, 
        if desired.
        """
        return EXT_PASS
    def before_insert(self, mapper, connection, instance):
        """receive an object instance before that instance is INSERTed into its table.
        
        this is a good place to set up primary key values and such that arent handled otherwise."""
        return EXT_PASS
    def before_update(self, mapper, connection, instance):
        """receive an object instance before that instance is UPDATEed."""
        return EXT_PASS
    def after_update(self, mapper, connection, instance):
        """receive an object instance after that instance is UPDATEed."""
        return EXT_PASS
    def after_insert(self, mapper, connection, instance):
        """receive an object instance after that instance is INSERTed."""
        return EXT_PASS
    def before_delete(self, mapper, connection, instance):
        """receive an object instance before that instance is DELETEed."""
        return EXT_PASS
    def after_delete(self, mapper, connection, instance):
        """receive an object instance after that instance is DELETEed."""
        return EXT_PASS

class _ExtensionCarrier(MapperExtension):
    def __init__(self):
        self.__elements = []
        self.__callables = {}
    def insert(self, extension):
        """insert a MapperExtension at the beginning of this ExtensionCarrier's list."""
        self.__elements.insert(0, extension)
    def append(self, extension):
        """append a MapperExtension at the end of this ExtensionCarrier's list."""
        self.__elements.append(extension)
    def __getattribute__(self, key):
        if key in MapperExtension.__dict__:
            try:
                return self.__callables[key]
            except KeyError:
                return self.__callables.setdefault(key, lambda *args, **kwargs:self._do(key, *args, **kwargs))
        else:
            return super(_ExtensionCarrier, self).__getattribute__(key)
    def _do(self, funcname, *args, **kwargs):
        for elem in self.__elements:
            if elem is self:
                raise exceptions.AssertionError("ExtensionCarrier set to itself")
            ret = getattr(elem, funcname)(*args, **kwargs)
            if ret is not EXT_PASS:
                return ret
        else:
            return EXT_PASS
    
class ExtensionOption(MapperExtension):
    def __init__(self, ext):
        self.ext = ext
    def process_query(self, query):
        query._insert_extension(self.ext)
                    
class ClassKey(object):
    """keys a class and an entity name to a mapper, via the mapper_registry."""
    __metaclass__ = util.ArgSingleton
    def __init__(self, class_, entity_name):
        self.class_ = class_
        self.entity_name = entity_name
    def __hash__(self):
        return hash((self.class_, self.entity_name))
    def __eq__(self, other):
        return self is other
    def __repr__(self):
        return "ClassKey(%s, %s)" % (repr(self.class_), repr(self.entity_name))

def has_identity(object):
    return hasattr(object, '_instance_key')
    
def has_mapper(object):
    """returns True if the given object has a mapper association"""
    return hasattr(object, '_entity_name')


        
def object_mapper(object, raiseerror=True):
    """given an object, returns the primary Mapper associated with the object instance"""
    try:
        mapper = mapper_registry[ClassKey(object.__class__, getattr(object, '_entity_name', None))]
    except (KeyError, AttributeError):        
        if raiseerror:
            raise exceptions.InvalidRequestError("Class '%s' entity name '%s' has no mapper associated with it" % (object.__class__.__name__, getattr(object, '_entity_name', None)))
        else:
            return None
    return mapper.compile()
    
def class_mapper(class_, entity_name=None, compile=True):
    """given a ClassKey, returns the primary Mapper associated with the key."""
    try:
        mapper = mapper_registry[ClassKey(class_, entity_name)]
    except (KeyError, AttributeError):
        raise exceptions.InvalidRequestError("Class '%s' entity name '%s' has no mapper associated with it" % (class_.__name__, entity_name))
    if compile:
        return mapper.compile()
    else:
        return mapper