path: root/lib/sqlalchemy/ext/orderinglist.py

"""A custom list that manages index/position information for its children.

``orderinglist`` is a custom list collection implementation for mapped relations
that keeps an arbitrary "position" attribute on contained objects in sync with
each object's position in the Python list.

The collection acts just like a normal Python ``list``, with the added
behavior that as you manipulate the list (via ``insert``, ``pop``, assignment,
deletion, what have you), each of the objects it contains is updated as needed
to reflect its position.  This is very useful for managing ordered relations
which have a user-defined, serialized order::

  from sqlalchemy.ext.orderinglist import ordering_list

  users = Table('users', metadata,
                Column('id', Integer, primary_key=True))
  blurbs = Table('user_top_ten_list', metadata,
                 Column('id', Integer, primary_key=True),
                 Column('user_id', Integer, ForeignKey('users.id')),
                 Column('position', Integer),
                 Column('blurb', String(80)))

  class User(object): pass
  class Blurb(object):
      def __init__(self, blurb):
          self.blurb = blurb

  mapper(User, users, properties={
    'topten': relation(Blurb, collection_class=ordering_list('position'),
                       order_by=[blurbs.c.position])
  })
  mapper(Blurb, blurbs)

  u = User()
  u.topten.append(Blurb('Number one!'))
  u.topten.append(Blurb('Number two!'))
  
  # Like magic.
  assert [blurb.position for blurb in u.topten] == [0, 1]

  # The objects will be renumbered automaticaly after any list-changing
  # operation, for example an insert:
  u.topten.insert(1, Blurb('I am the new Number Two.'))

  assert [blurb.position for blurb in u.topten] == [0, 1, 2]
  assert u.topten[1].blurb == 'I am the new Number Two.'
  assert u.topten[1].position == 1

Numbering and serialization are both highly configurable.  See the docstrings
in this module and the main SQLAlchemy documentation for more information and
examples.

The [sqlalchemy.ext.orderinglist#ordering_list] function is the ORM-compatible
constructor for OrderingList instances.
"""


__all__ = [ 'ordering_list' ]


def ordering_list(attr, count_from=None, **kw):
    """Prepares an OrderingList factory for use in mapper definitions.
    
    Returns an object suitable for use as an argument to a Mapper relation's
    ``collection_class`` option.  Arguments are:

    attr
      Name of the mapped attribute to use for storage and retrieval of
      ordering information

    count_from (optional)
      Set up an integer-based ordering, starting at ``count_from``.  For
      example, ``ordering_list('pos', count_from=1)`` would create a 1-based
      list in SQL, storing the value in the 'pos' column.  Ignored if
      ``ordering_func`` is supplied.
      
    Passes along any keyword arguments to ``OrderingList`` constructor.
    """

    kw = _unsugar_count_from(count_from=count_from, **kw)
    return lambda: OrderingList(attr, **kw)

# Ordering utility functions
def count_from_0(index, collection):
    """Numbering function: consecutive integers starting at 0."""

    return index

def count_from_1(index, collection):
    """Numbering function: consecutive integers starting at 1."""

    return index + 1

def count_from_n_factory(start):
    """Numbering function: consecutive integers starting at arbitrary start."""

    def f(index, collection):
        return index + start
    try:
        f.__name__ = 'count_from_%i' % start
    except TypeError:
        pass
    return f

def _unsugar_count_from(**kw):
    """Builds counting functions from keywrod arguments.

    Keyword argument filter, prepares a simple ``ordering_func`` from a
    ``count_from`` argument, otherwise passes ``ordering_func`` on unchanged.
    """
    
    count_from = kw.pop('count_from', None)
    if kw.get('ordering_func', None) is None and count_from is not None:
        if count_from == 0:
            kw['ordering_func'] = count_from_0
        elif count_from == 1:
            kw['ordering_func'] = count_from_1
        else:
            kw['ordering_func'] = count_from_n_factory(count_from)
    return kw

class OrderingList(list):
    """A custom list that manages position information for its children.

    See the module and __init__ documentation for more details.  The
    ``ordering_list`` function is used to configure ``OrderingList``
    collections in ``mapper`` relation definitions.
    """
    
    def __init__(self, ordering_attr=None, ordering_func=None,
                 reorder_on_append=False):
        """A custom list that manages position information for its children.
        
        ``OrderingList`` is a ``collection_class`` list implementation that
        syncs position in a Python list with a position attribute on the
        mapped objects.

        This implementation relies on the list starting in the proper order,
        so be **sure** to put an ``order_by`` on your relation.

        ordering_attr
          Name of the attribute that stores the object's order in the relation.

        ordering_func
          Optional.  A function that maps the position in the Python list to a
          value to store in the ``ordering_attr``.  Values returned are usually
          (but need not be!) integers.

          An ``ordering_func`` is called with two positional parameters: the
          index of the element in the list, and the list itself.
          
          If omitted, Python list indexes are used for the attribute values.
          Two basic pre-built numbering functions are provided in this module:
          ``count_from_0`` and ``count_from_1``.  For more exotic examples
          like stepped numbering, alphabetical and Fibonacci numbering, see
          the unit tests.

        reorder_on_append
          Default False.  When appending an object with an existing (non-None)
          ordering value, that value will be left untouched unless
          ``reorder_on_append`` is true.  This is an optimization to avoid a
          variety of dangerous unexpected database writes.

          SQLAlchemy will add instances to the list via append() when your
          object loads.  If for some reason the result set from the database
          skips a step in the ordering (say, row '1' is missing but you get
          '2', '3', and '4'), reorder_on_append=True would immediately
          renumber the items to '1', '2', '3'.  If you have multiple sessions
          making changes, any of whom happen to load this collection even in
          passing, all of the sessions would try to "clean up" the numbering
          in their commits, possibly causing all but one to fail with a
          concurrent modification error.  Spooky action at a distance.

          Recommend leaving this with the default of False, and just call
          ``_reorder()`` if you're doing ``append()`` operations with
          previously ordered instances or when doing some housekeeping after
          manual sql operations.
        """

        self.ordering_attr = ordering_attr
        if ordering_func is None:
            ordering_func = count_from_0
        self.ordering_func = ordering_func
        self.reorder_on_append = reorder_on_append

    # More complex serialization schemes (multi column, e.g.) are possible by
    # subclassing and reimplementing these two methods.
    def _get_order_value(self, entity):
        return getattr(entity, self.ordering_attr)

    def _set_order_value(self, entity, value):
        setattr(entity, self.ordering_attr, value)

    def _reorder(self):
        """Sweep through the list and ensure that each object has accurate
        ordering information set."""
        
        for index, entity in enumerate(self):
            self._order_entity(index, entity, True)

    def _order_entity(self, index, entity, reorder=True):
        have = self._get_order_value(entity)

        # Don't disturb existing ordering if reorder is False
        if have is not None and not reorder:
            return

        should_be = self.ordering_func(index, self)
        if have <> should_be:
            self._set_order_value(entity, should_be)

    def append(self, entity):
        super(OrderingList, self).append(entity)
        self._order_entity(len(self) - 1, entity, self.reorder_on_append)

    def _raw_append(self, entity):
        """Append without any ordering behavior."""

        super(OrderingList, self).append(entity)

    def insert(self, index, entity):
        self[index:index] = [entity]

    def remove(self, entity):
        super(OrderingList, self).remove(entity)
        self._reorder()

    def pop(self, index=-1):
        entity = super(OrderingList, self).pop(index)
        self._reorder()
        return entity
        
    def __setitem__(self, index, entity):
        if isinstance(index, slice):
            for i in range(index.start or 0, index.stop or 0, index.step or 1):
                self.__setitem__(i, entity[i])
        else:
            self._order_entity(index, entity, True)
            super(OrderingList, self).__setitem__(index, entity)
            
    def __delitem__(self, index):
        super(OrderingList, self).__delitem__(index)
        self._reorder()

    def __setslice__(self, start, end, values):
        super(OrderingList, self).__setslice__(start, end, values)
        self._reorder()

    def __delslice__(self, start, end):
        super(OrderingList, self).__delslice__(start, end)
        self._reorder()