"""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 import MetaData, Table, Column, Integer, String, ForeignKey >>> from sqlalchemy.orm import mapper, relation >>> from sqlalchemy.ext.orderinglist import ordering_list A simple model of users their "top 10" things:: >>> metadata = MetaData() >>> 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) Acts just like a regular list:: >>> u = User() >>> u.topten.append(Blurb('Number one!')) >>> u.topten.append(Blurb('Number two!')) But the ``.position`` attibute is set automatically behind the scenes:: >>> 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 :class:`~sqlalchemy.ext.orderinglist.ordering_list` factory function is the ORM-compatible constructor for `OrderingList` instances. """ from sqlalchemy.orm.collections import collection from sqlalchemy import util __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`` factory 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): """Synchronize ordering for the entire collection. Sweeps through the list and ensures that each object has accurate ordering information set. """ for index, entity in enumerate(self): self._order_entity(index, entity, True) # As of 0.5, _reorder is no longer semi-private _reorder = reorder 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) _raw_append = collection.adds(1)(_raw_append) 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() for func_name, func in locals().items(): if (util.callable(func) and func.func_name == func_name and not func.__doc__ and hasattr(list, func_name)): func.__doc__ = getattr(list, func_name).__doc__ del func_name, func if __name__ == '__main__': import doctest doctest.testmod(optionflags=doctest.ELLIPSIS)