# SQLAlchemy 0.4 Documentation

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Version: 0.4.6 Last Updated: 05/14/08 14:36:43

### module sqlalchemy.ext.orderinglist

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 ordering_list function is the ORM-compatible constructor for OrderingList instances.

### Module Functions

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.

### 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.

def append(self, entity)
def insert(self, index, entity)
def pop(self, index=-1)
def remove(self, entity)
def __delitem__(self, index)
def __delslice__(self, start, end)
def __setitem__(self, index, entity)
def __setslice__(self, start, end, values)
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