path: root/django/db/models/query.py

try:
    set
except NameError:
    from sets import Set as set     # Python 2.3 fallback

from django.db import connection, transaction, IntegrityError
from django.db.models.aggregates import Aggregate
from django.db.models.fields import DateField
from django.db.models.query_utils import Q, select_related_descend
from django.db.models import signals, sql
from django.utils.datastructures import SortedDict


# Used to control how many objects are worked with at once in some cases (e.g.
# when deleting objects).
CHUNK_SIZE = 100
ITER_CHUNK_SIZE = CHUNK_SIZE

# The maximum number of items to display in a QuerySet.__repr__
REPR_OUTPUT_SIZE = 20

# Pull into this namespace for backwards compatibility.
EmptyResultSet = sql.EmptyResultSet


class CyclicDependency(Exception):
    """
    An error when dealing with a collection of objects that have a cyclic
    dependency, i.e. when deleting multiple objects.
    """
    pass


class CollectedObjects(object):
    """
    A container that stores keys and lists of values along with remembering the
    parent objects for all the keys.

    This is used for the database object deletion routines so that we can
    calculate the 'leaf' objects which should be deleted first.
    """

    def __init__(self):
        self.data = {}
        self.children = {}

    def add(self, model, pk, obj, parent_model, nullable=False):
        """
        Adds an item to the container.

        Arguments:
        * model - the class of the object being added.
        * pk - the primary key.
        * obj - the object itself.
        * parent_model - the model of the parent object that this object was
          reached through.
        * nullable - should be True if this relation is nullable.

        Returns True if the item already existed in the structure and
        False otherwise.
        """
        d = self.data.setdefault(model, SortedDict())
        retval = pk in d
        d[pk] = obj
        # Nullable relationships can be ignored -- they are nulled out before
        # deleting, and therefore do not affect the order in which objects
        # have to be deleted.
        if parent_model is not None and not nullable:
            self.children.setdefault(parent_model, []).append(model)
        return retval

    def __contains__(self, key):
        return self.data.__contains__(key)

    def __getitem__(self, key):
        return self.data[key]

    def __nonzero__(self):
        return bool(self.data)

    def iteritems(self):
        for k in self.ordered_keys():
            yield k, self[k]

    def items(self):
        return list(self.iteritems())

    def keys(self):
        return self.ordered_keys()

    def ordered_keys(self):
        """
        Returns the models in the order that they should be dealt with (i.e.
        models with no dependencies first).
        """
        dealt_with = SortedDict()
        # Start with items that have no children
        models = self.data.keys()
        while len(dealt_with) < len(models):
            found = False
            for model in models:
                if model in dealt_with:
                    continue
                children = self.children.setdefault(model, [])
                if len([c for c in children if c not in dealt_with]) == 0:
                    dealt_with[model] = None
                    found = True
            if not found:
                raise CyclicDependency(
                    "There is a cyclic dependency of items to be processed.")

        return dealt_with.keys()

    def unordered_keys(self):
        """
        Fallback for the case where is a cyclic dependency but we don't  care.
        """
        return self.data.keys()


class QuerySet(object):
    """
    Represents a lazy database lookup for a set of objects.
    """
    def __init__(self, model=None, query=None):
        self.model = model
        self.query = query or sql.Query(self.model, connection)
        self._result_cache = None
        self._iter = None
        self._sticky_filter = False

    ########################
    # PYTHON MAGIC METHODS #
    ########################

    def __getstate__(self):
        """
        Allows the QuerySet to be pickled.
        """
        # Force the cache to be fully populated.
        len(self)

        obj_dict = self.__dict__.copy()
        obj_dict['_iter'] = None
        return obj_dict

    def __repr__(self):
        data = list(self[:REPR_OUTPUT_SIZE + 1])
        if len(data) > REPR_OUTPUT_SIZE:
            data[-1] = "...(remaining elements truncated)..."
        return repr(data)

    def __len__(self):
        # Since __len__ is called quite frequently (for example, as part of
        # list(qs), we make some effort here to be as efficient as possible
        # whilst not messing up any existing iterators against the QuerySet.
        if self._result_cache is None:
            if self._iter:
                self._result_cache = list(self._iter)
            else:
                self._result_cache = list(self.iterator())
        elif self._iter:
            self._result_cache.extend(list(self._iter))
        return len(self._result_cache)

    def __iter__(self):
        if self._result_cache is None:
            self._iter = self.iterator()
            self._result_cache = []
        if self._iter:
            return self._result_iter()
        # Python's list iterator is better than our version when we're just
        # iterating over the cache.
        return iter(self._result_cache)

    def _result_iter(self):
        pos = 0
        while 1:
            upper = len(self._result_cache)
            while pos < upper:
                yield self._result_cache[pos]
                pos = pos + 1
            if not self._iter:
                raise StopIteration
            if len(self._result_cache) <= pos:
                self._fill_cache()

    def __nonzero__(self):
        if self._result_cache is not None:
            return bool(self._result_cache)
        try:
            iter(self).next()
        except StopIteration:
            return False
        return True

    def __getitem__(self, k):
        """
        Retrieves an item or slice from the set of results.
        """
        if not isinstance(k, (slice, int, long)):
            raise TypeError
        assert ((not isinstance(k, slice) and (k >= 0))
                or (isinstance(k, slice) and (k.start is None or k.start >= 0)
                    and (k.stop is None or k.stop >= 0))), \
                "Negative indexing is not supported."

        if self._result_cache is not None:
            if self._iter is not None:
                # The result cache has only been partially populated, so we may
                # need to fill it out a bit more.
                if isinstance(k, slice):
                    if k.stop is not None:
                        # Some people insist on passing in strings here.
                        bound = int(k.stop)
                    else:
                        bound = None
                else:
                    bound = k + 1
                if len(self._result_cache) < bound:
                    self._fill_cache(bound - len(self._result_cache))
            return self._result_cache[k]

        if isinstance(k, slice):
            qs = self._clone()
            if k.start is not None:
                start = int(k.start)
            else:
                start = None
            if k.stop is not None:
                stop = int(k.stop)
            else:
                stop = None
            qs.query.set_limits(start, stop)
            return k.step and list(qs)[::k.step] or qs
        try:
            qs = self._clone()
            qs.query.set_limits(k, k + 1)
            return list(qs)[0]
        except self.model.DoesNotExist, e:
            raise IndexError, e.args

    def __and__(self, other):
        self._merge_sanity_check(other)
        if isinstance(other, EmptyQuerySet):
            return other._clone()
        combined = self._clone()
        combined.query.combine(other.query, sql.AND)
        return combined

    def __or__(self, other):
        self._merge_sanity_check(other)
        combined = self._clone()
        if isinstance(other, EmptyQuerySet):
            return combined
        combined.query.combine(other.query, sql.OR)
        return combined

    ####################################
    # METHODS THAT DO DATABASE QUERIES #
    ####################################

    def iterator(self):
        """
        An iterator over the results from applying this QuerySet to the
        database.
        """
        fill_cache = self.query.select_related
        if isinstance(fill_cache, dict):
            requested = fill_cache
        else:
            requested = None
        max_depth = self.query.max_depth

        extra_select = self.query.extra_select.keys()
        aggregate_select = self.query.aggregate_select.keys()

        index_start = len(extra_select)
        aggregate_start = index_start + len(self.model._meta.fields)

        for row in self.query.results_iter():
            if fill_cache:
                obj, _ = get_cached_row(self.model, row,
                            index_start, max_depth,
                            requested=requested, offset=len(aggregate_select))
            else:
                # omit aggregates in object creation
                obj = self.model(*row[index_start:aggregate_start])

            for i, k in enumerate(extra_select):
                setattr(obj, k, row[i])

            # Add the aggregates to the model
            for i, aggregate in enumerate(aggregate_select):
                setattr(obj, aggregate, row[i+aggregate_start])

            yield obj

    def aggregate(self, *args, **kwargs):
        """
        Returns a dictionary containing the calculations (aggregation)
        over the current queryset

        If args is present the expression is passed as a kwarg ussing
        the Aggregate object's default alias.
        """
        for arg in args:
            kwargs[arg.default_alias] = arg

        query = self.query.clone()

        for (alias, aggregate_expr) in kwargs.items():
            query.add_aggregate(aggregate_expr, self.model, alias,
                is_summary=True)

        return query.get_aggregation()

    def count(self):
        """
        Performs a SELECT COUNT() and returns the number of records as an
        integer.

        If the QuerySet is already fully cached this simply returns the length
        of the cached results set to avoid multiple SELECT COUNT(*) calls.
        """
        if self._result_cache is not None and not self._iter:
            return len(self._result_cache)

        return self.query.get_count()

    def get(self, *args, **kwargs):
        """
        Performs the query and returns a single object matching the given
        keyword arguments.
        """
        clone = self.filter(*args, **kwargs)
        num = len(clone)
        if num == 1:
            return clone._result_cache[0]
        if not num:
            raise self.model.DoesNotExist("%s matching query does not exist."
                    % self.model._meta.object_name)
        raise self.model.MultipleObjectsReturned("get() returned more than one %s -- it returned %s! Lookup parameters were %s"
                % (self.model._meta.object_name, num, kwargs))

    def create(self, **kwargs):
        """
        Creates a new object with the given kwargs, saving it to the database
        and returning the created object.
        """
        obj = self.model(**kwargs)
        obj.save(force_insert=True)
        return obj

    def get_or_create(self, **kwargs):
        """
        Looks up an object with the given kwargs, creating one if necessary.
        Returns a tuple of (object, created), where created is a boolean
        specifying whether an object was created.
        """
        assert kwargs, \
                'get_or_create() must be passed at least one keyword argument'
        defaults = kwargs.pop('defaults', {})
        try:
            return self.get(**kwargs), False
        except self.model.DoesNotExist:
            try:
                params = dict([(k, v) for k, v in kwargs.items() if '__' not in k])
                params.update(defaults)
                obj = self.model(**params)
                sid = transaction.savepoint()
                obj.save(force_insert=True)
                transaction.savepoint_commit(sid)
                return obj, True
            except IntegrityError, e:
                transaction.savepoint_rollback(sid)
                try:
                    return self.get(**kwargs), False
                except self.model.DoesNotExist:
                    raise e

    def latest(self, field_name=None):
        """
        Returns the latest object, according to the model's 'get_latest_by'
        option or optional given field_name.
        """
        latest_by = field_name or self.model._meta.get_latest_by
        assert bool(latest_by), "latest() requires either a field_name parameter or 'get_latest_by' in the model"
        assert self.query.can_filter(), \
                "Cannot change a query once a slice has been taken."
        obj = self._clone()
        obj.query.set_limits(high=1)
        obj.query.add_ordering('-%s' % latest_by)
        return obj.get()

    def in_bulk(self, id_list):
        """
        Returns a dictionary mapping each of the given IDs to the object with
        that ID.
        """
        assert self.query.can_filter(), \
                "Cannot use 'limit' or 'offset' with in_bulk"
        assert isinstance(id_list, (tuple,  list)), \
                "in_bulk() must be provided with a list of IDs."
        if not id_list:
            return {}
        qs = self._clone()
        qs.query.add_filter(('pk__in', id_list))
        return dict([(obj._get_pk_val(), obj) for obj in qs.iterator()])

    def delete(self):
        """
        Deletes the records in the current QuerySet.
        """
        assert self.query.can_filter(), \
                "Cannot use 'limit' or 'offset' with delete."

        del_query = self._clone()

        # Disable non-supported fields.
        del_query.query.select_related = False
        del_query.query.clear_ordering()

        # Delete objects in chunks to prevent the list of related objects from
        # becoming too long.
        while 1:
            # Collect all the objects to be deleted in this chunk, and all the
            # objects that are related to the objects that are to be deleted.
            seen_objs = CollectedObjects()
            for object in del_query[:CHUNK_SIZE]:
                object._collect_sub_objects(seen_objs)

            if not seen_objs:
                break
            delete_objects(seen_objs)

        # Clear the result cache, in case this QuerySet gets reused.
        self._result_cache = None
    delete.alters_data = True

    def update(self, **kwargs):
        """
        Updates all elements in the current QuerySet, setting all the given
        fields to the appropriate values.
        """
        assert self.query.can_filter(), \
                "Cannot update a query once a slice has been taken."
        query = self.query.clone(sql.UpdateQuery)
        query.add_update_values(kwargs)
        rows = query.execute_sql(None)
        transaction.commit_unless_managed()
        self._result_cache = None
        return rows
    update.alters_data = True

    def _update(self, values):
        """
        A version of update that accepts field objects instead of field names.
        Used primarily for model saving and not intended for use by general
        code (it requires too much poking around at model internals to be
        useful at that level).
        """
        assert self.query.can_filter(), \
                "Cannot update a query once a slice has been taken."
        query = self.query.clone(sql.UpdateQuery)
        query.add_update_fields(values)
        self._result_cache = None
        return query.execute_sql(None)
    _update.alters_data = True

    ##################################################
    # PUBLIC METHODS THAT RETURN A QUERYSET SUBCLASS #
    ##################################################

    def values(self, *fields):
        return self._clone(klass=ValuesQuerySet, setup=True, _fields=fields)

    def values_list(self, *fields, **kwargs):
        flat = kwargs.pop('flat', False)
        if kwargs:
            raise TypeError('Unexpected keyword arguments to values_list: %s'
                    % (kwargs.keys(),))
        if flat and len(fields) > 1:
            raise TypeError("'flat' is not valid when values_list is called with more than one field.")
        return self._clone(klass=ValuesListQuerySet, setup=True, flat=flat,
                _fields=fields)

    def dates(self, field_name, kind, order='ASC'):
        """
        Returns a list of datetime objects representing all available dates for
        the given field_name, scoped to 'kind'.
        """
        assert kind in ("month", "year", "day"), \
                "'kind' must be one of 'year', 'month' or 'day'."
        assert order in ('ASC', 'DESC'), \
                "'order' must be either 'ASC' or 'DESC'."
        return self._clone(klass=DateQuerySet, setup=True,
                _field_name=field_name, _kind=kind, _order=order)

    def none(self):
        """
        Returns an empty QuerySet.
        """
        return self._clone(klass=EmptyQuerySet)

    ##################################################################
    # PUBLIC METHODS THAT ALTER ATTRIBUTES AND RETURN A NEW QUERYSET #
    ##################################################################

    def all(self):
        """
        Returns a new QuerySet that is a copy of the current one. This allows a
        QuerySet to proxy for a model manager in some cases.
        """
        return self._clone()

    def filter(self, *args, **kwargs):
        """
        Returns a new QuerySet instance with the args ANDed to the existing
        set.
        """
        return self._filter_or_exclude(False, *args, **kwargs)

    def exclude(self, *args, **kwargs):
        """
        Returns a new QuerySet instance with NOT (args) ANDed to the existing
        set.
        """
        return self._filter_or_exclude(True, *args, **kwargs)

    def _filter_or_exclude(self, negate, *args, **kwargs):
        if args or kwargs:
            assert self.query.can_filter(), \
                    "Cannot filter a query once a slice has been taken."

        clone = self._clone()
        if negate:
            clone.query.add_q(~Q(*args, **kwargs))
        else:
            clone.query.add_q(Q(*args, **kwargs))
        return clone

    def complex_filter(self, filter_obj):
        """
        Returns a new QuerySet instance with filter_obj added to the filters.

        filter_obj can be a Q object (or anything with an add_to_query()
        method) or a dictionary of keyword lookup arguments.

        This exists to support framework features such as 'limit_choices_to',
        and usually it will be more natural to use other methods.
        """
        if isinstance(filter_obj, Q) or hasattr(filter_obj, 'add_to_query'):
            clone = self._clone()
            clone.query.add_q(filter_obj)
            return clone
        else:
            return self._filter_or_exclude(None, **filter_obj)

    def select_related(self, *fields, **kwargs):
        """
        Returns a new QuerySet instance that will select related objects.

        If fields are specified, they must be ForeignKey fields and only those
        related objects are included in the selection.
        """
        depth = kwargs.pop('depth', 0)
        if kwargs:
            raise TypeError('Unexpected keyword arguments to select_related: %s'
                    % (kwargs.keys(),))
        obj = self._clone()
        if fields:
            if depth:
                raise TypeError('Cannot pass both "depth" and fields to select_related()')
            obj.query.add_select_related(fields)
        else:
            obj.query.select_related = True
        if depth:
            obj.query.max_depth = depth
        return obj

    def dup_select_related(self, other):
        """
        Copies the related selection status from the QuerySet 'other' to the
        current QuerySet.
        """
        self.query.select_related = other.query.select_related

    def annotate(self, *args, **kwargs):
        """
        Return a query set in which the returned objects have been annotated
        with data aggregated from related fields.
        """
        for arg in args:
            kwargs[arg.default_alias] = arg

        obj = self._clone()

        obj._setup_aggregate_query(kwargs.keys())

        # Add the aggregates to the query
        for (alias, aggregate_expr) in kwargs.items():
            obj.query.add_aggregate(aggregate_expr, self.model, alias,
                is_summary=False)

        return obj

    def order_by(self, *field_names):
        """
        Returns a new QuerySet instance with the ordering changed.
        """
        assert self.query.can_filter(), \
                "Cannot reorder a query once a slice has been taken."
        obj = self._clone()
        obj.query.clear_ordering()
        obj.query.add_ordering(*field_names)
        return obj

    def distinct(self, true_or_false=True):
        """
        Returns a new QuerySet instance that will select only distinct results.
        """
        obj = self._clone()
        obj.query.distinct = true_or_false
        return obj

    def extra(self, select=None, where=None, params=None, tables=None,
              order_by=None, select_params=None):
        """
        Adds extra SQL fragments to the query.
        """
        assert self.query.can_filter(), \
                "Cannot change a query once a slice has been taken"
        clone = self._clone()
        clone.query.add_extra(select, select_params, where, params, tables, order_by)
        return clone

    def reverse(self):
        """
        Reverses the ordering of the QuerySet.
        """
        clone = self._clone()
        clone.query.standard_ordering = not clone.query.standard_ordering
        return clone

    ###################
    # PRIVATE METHODS #
    ###################

    def _clone(self, klass=None, setup=False, **kwargs):
        if klass is None:
            klass = self.__class__
        query = self.query.clone()
        if self._sticky_filter:
            query.filter_is_sticky = True
        c = klass(model=self.model, query=query)
        c.__dict__.update(kwargs)
        if setup and hasattr(c, '_setup_query'):
            c._setup_query()
        return c

    def _fill_cache(self, num=None):
        """
        Fills the result cache with 'num' more entries (or until the results
        iterator is exhausted).
        """
        if self._iter:
            try:
                for i in range(num or ITER_CHUNK_SIZE):
                    self._result_cache.append(self._iter.next())
            except StopIteration:
                self._iter = None

    def _next_is_sticky(self):
        """
        Indicates that the next filter call and the one following that should
        be treated as a single filter. This is only important when it comes to
        determining when to reuse tables for many-to-many filters. Required so
        that we can filter naturally on the results of related managers.

        This doesn't return a clone of the current QuerySet (it returns
        "self"). The method is only used internally and should be immediately
        followed by a filter() that does create a clone.
        """
        self._sticky_filter = True
        return self

    def _merge_sanity_check(self, other):
        """
        Checks that we are merging two comparable QuerySet classes. By default
        this does nothing, but see the ValuesQuerySet for an example of where
        it's useful.
        """
        pass

    def _setup_aggregate_query(self, aggregates):
        """
        Prepare the query for computing a result that contains aggregate annotations.
        """
        opts = self.model._meta
        if self.query.group_by is None:
            field_names = [f.attname for f in opts.fields]
            self.query.add_fields(field_names, False)
            self.query.set_group_by()

    def as_sql(self):
        """
        Returns the internal query's SQL and parameters (as a tuple).

        This is a private (internal) method. The name is chosen to provide
        uniformity with other interfaces (in particular, the Query class).
        """
        obj = self.values("pk")
        return obj.query.as_nested_sql()

    # When used as part of a nested query, a queryset will never be an "always
    # empty" result.
    value_annotation = True

class ValuesQuerySet(QuerySet):
    def __init__(self, *args, **kwargs):
        super(ValuesQuerySet, self).__init__(*args, **kwargs)
        # select_related isn't supported in values(). (FIXME -#3358)
        self.query.select_related = False

        # QuerySet.clone() will also set up the _fields attribute with the
        # names of the model fields to select.

    def iterator(self):
        # Purge any extra columns that haven't been explicitly asked for
        if self.extra_names is not None:
            self.query.trim_extra_select(self.extra_names)

        extra_names = self.query.extra_select.keys()
        field_names = self.field_names
        aggregate_names = self.query.aggregate_select.keys()

        names = extra_names + field_names + aggregate_names

        for row in self.query.results_iter():
            yield dict(zip(names, row))

    def _setup_query(self):
        """
        Constructs the field_names list that the values query will be
        retrieving.

        Called by the _clone() method after initializing the rest of the
        instance.
        """
        self.query.clear_select_fields()

        if self._fields:
            self.extra_names = []
            self.aggregate_names = []
            if not self.query.extra_select and not self.query.aggregate_select:
                self.field_names = list(self._fields)
            else:
                self.query.default_cols = False
                self.field_names = []
                for f in self._fields:
                    if self.query.extra_select.has_key(f):
                        self.extra_names.append(f)
                    elif self.query.aggregate_select.has_key(f):
                        self.aggregate_names.append(f)
                    else:
                        self.field_names.append(f)
        else:
            # Default to all fields.
            self.extra_names = None
            self.field_names = [f.attname for f in self.model._meta.fields]
            self.aggregate_names = None

        self.query.select = []
        self.query.add_fields(self.field_names, False)
        if self.aggregate_names is not None:
            self.query.set_aggregate_mask(self.aggregate_names)

    def _clone(self, klass=None, setup=False, **kwargs):
        """
        Cloning a ValuesQuerySet preserves the current fields.
        """
        c = super(ValuesQuerySet, self)._clone(klass, **kwargs)
        if not hasattr(c, '_fields'):
            # Only clone self._fields if _fields wasn't passed into the cloning
            # call directly.
            c._fields = self._fields[:]
        c.field_names = self.field_names
        c.extra_names = self.extra_names
        c.aggregate_names = self.aggregate_names
        if setup and hasattr(c, '_setup_query'):
            c._setup_query()
        return c

    def _merge_sanity_check(self, other):
        super(ValuesQuerySet, self)._merge_sanity_check(other)
        if (set(self.extra_names) != set(other.extra_names) or
                set(self.field_names) != set(other.field_names) or
                self.aggregate_names != other.aggregate_names):
            raise TypeError("Merging '%s' classes must involve the same values in each case."
                    % self.__class__.__name__)

    def _setup_aggregate_query(self, aggregates):
        """
        Prepare the query for computing a result that contains aggregate annotations.
        """
        self.query.set_group_by()

        if self.aggregate_names is not None:
            self.aggregate_names.extend(aggregates)
            self.query.set_aggregate_mask(self.aggregate_names)

        super(ValuesQuerySet, self)._setup_aggregate_query(aggregates)

    def as_sql(self):
        """
        For ValueQuerySet (and subclasses like ValuesListQuerySet), they can
        only be used as nested queries if they're already set up to select only
        a single field (in which case, that is the field column that is
        returned). This differs from QuerySet.as_sql(), where the column to
        select is set up by Django.
        """
        if ((self._fields and len(self._fields) > 1) or
                (not self._fields and len(self.model._meta.fields) > 1)):
            raise TypeError('Cannot use a multi-field %s as a filter value.'
                    % self.__class__.__name__)
        return self._clone().query.as_nested_sql()

class ValuesListQuerySet(ValuesQuerySet):
    def iterator(self):
        if self.extra_names is not None:
            self.query.trim_extra_select(self.extra_names)

        if self.flat and len(self._fields) == 1:
            for row in self.query.results_iter():
                yield row[0]
        elif not self.query.extra_select and not self.query.aggregate_select:
            for row in self.query.results_iter():
                yield tuple(row)
        else:
            # When extra(select=...) or an annotation is involved, the extra cols are
            # always at the start of the row, and we need to reorder the fields
            # to match the order in self._fields.
            extra_names = self.query.extra_select.keys()
            field_names = self.field_names
            aggregate_names = self.query.aggregate_select.keys()

            names = extra_names + field_names + aggregate_names

            # If a field list has been specified, use it. Otherwise, use the
            # full list of fields, including extras and aggregates.
            if self._fields:
                fields = self._fields
            else:
                fields = names

            for row in self.query.results_iter():
                data = dict(zip(names, row))
                yield tuple([data[f] for f in fields])

    def _clone(self, *args, **kwargs):
        clone = super(ValuesListQuerySet, self)._clone(*args, **kwargs)
        clone.flat = self.flat
        return clone


class DateQuerySet(QuerySet):
    def iterator(self):
        return self.query.results_iter()

    def _setup_query(self):
        """
        Sets up any special features of the query attribute.

        Called by the _clone() method after initializing the rest of the
        instance.
        """
        self.query = self.query.clone(klass=sql.DateQuery, setup=True)
        self.query.select = []
        field = self.model._meta.get_field(self._field_name, many_to_many=False)
        assert isinstance(field, DateField), "%r isn't a DateField." \
                % field.name
        self.query.add_date_select(field, self._kind, self._order)
        if field.null:
            self.query.add_filter(('%s__isnull' % field.name, False))

    def _clone(self, klass=None, setup=False, **kwargs):
        c = super(DateQuerySet, self)._clone(klass, False, **kwargs)
        c._field_name = self._field_name
        c._kind = self._kind
        if setup and hasattr(c, '_setup_query'):
            c._setup_query()
        return c


class EmptyQuerySet(QuerySet):
    def __init__(self, model=None, query=None):
        super(EmptyQuerySet, self).__init__(model, query)
        self._result_cache = []

    def __and__(self, other):
        return self._clone()

    def __or__(self, other):
        return other._clone()

    def count(self):
        return 0

    def delete(self):
        pass

    def _clone(self, klass=None, setup=False, **kwargs):
        c = super(EmptyQuerySet, self)._clone(klass, **kwargs)
        c._result_cache = []
        return c

    def iterator(self):
        # This slightly odd construction is because we need an empty generator
        # (it raises StopIteration immediately).
        yield iter([]).next()

    # EmptyQuerySet is always an empty result in where-clauses (and similar
    # situations).
    value_annotation = False


def get_cached_row(klass, row, index_start, max_depth=0, cur_depth=0,
                   requested=None, offset=0):
    """
    Helper function that recursively returns an object with the specified
    related attributes already populated.
    """
    if max_depth and requested is None and cur_depth > max_depth:
        # We've recursed deeply enough; stop now.
        return None

    restricted = requested is not None
    index_end = index_start + len(klass._meta.fields)
    fields = row[index_start:index_end]
    if not [x for x in fields if x is not None]:
        # If we only have a list of Nones, there was not related object.
        obj = None
    else:
        obj = klass(*fields)
    index_end += offset
    for f in klass._meta.fields:
        if not select_related_descend(f, restricted, requested):
            continue
        if restricted:
            next = requested[f.name]
        else:
            next = None
        cached_row = get_cached_row(f.rel.to, row, index_end, max_depth,
                cur_depth+1, next)
        if cached_row:
            rel_obj, index_end = cached_row
            if obj is not None:
                setattr(obj, f.get_cache_name(), rel_obj)
    return obj, index_end


def delete_objects(seen_objs):
    """
    Iterate through a list of seen classes, and remove any instances that are
    referred to.
    """
    try:
        ordered_classes = seen_objs.keys()
    except CyclicDependency:
        # If there is a cyclic dependency, we cannot in general delete the
        # objects.  However, if an appropriate transaction is set up, or if the
        # database is lax enough, it will succeed. So for now, we go ahead and
        # try anyway.
        ordered_classes = seen_objs.unordered_keys()

    obj_pairs = {}
    for cls in ordered_classes:
        items = seen_objs[cls].items()
        items.sort()
        obj_pairs[cls] = items

        # Pre-notify all instances to be deleted.
        for pk_val, instance in items:
            signals.pre_delete.send(sender=cls, instance=instance)

        pk_list = [pk for pk,instance in items]
        del_query = sql.DeleteQuery(cls, connection)
        del_query.delete_batch_related(pk_list)

        update_query = sql.UpdateQuery(cls, connection)
        for field, model in cls._meta.get_fields_with_model():
            if (field.rel and field.null and field.rel.to in seen_objs and
                    filter(lambda f: f.column == field.column,
                    field.rel.to._meta.fields)):
                if model:
                    sql.UpdateQuery(model, connection).clear_related(field,
                            pk_list)
                else:
                    update_query.clear_related(field, pk_list)

    # Now delete the actual data.
    for cls in ordered_classes:
        items = obj_pairs[cls]
        items.reverse()

        pk_list = [pk for pk,instance in items]
        del_query = sql.DeleteQuery(cls, connection)
        del_query.delete_batch(pk_list)

        # Last cleanup; set NULLs where there once was a reference to the
        # object, NULL the primary key of the found objects, and perform
        # post-notification.
        for pk_val, instance in items:
            for field in cls._meta.fields:
                if field.rel and field.null and field.rel.to in seen_objs:
                    setattr(instance, field.attname, None)

            signals.post_delete.send(sender=cls, instance=instance)
            setattr(instance, cls._meta.pk.attname, None)

    transaction.commit_unless_managed()


def insert_query(model, values, return_id=False, raw_values=False):
    """
    Inserts a new record for the given model. This provides an interface to
    the InsertQuery class and is how Model.save() is implemented. It is not
    part of the public API.
    """
    query = sql.InsertQuery(model, connection)
    query.insert_values(values, raw_values)
    return query.execute_sql(return_id)