# SQLAlchemy 0.7 Documentation

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Release: 0.7.8 | Release Date: June 16, 2012

# Ordering List

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

author: | Jason Kirtland

`orderinglist` is a helper for mutable ordered relationships. It will intercept list operations performed on a relationship collection and automatically synchronize changes in list position with an attribute on the related objects. (See advdatamapping_entitycollections for more information on the general pattern.)

Example: Two tables that store slides in a presentation. Each slide has a number of bullet points, displayed in order by the ‘position’ column on the bullets table. These bullets can be inserted and re-ordered by your end users, and you need to update the ‘position’ column of all affected rows when changes are made.

    slides_table = Table('Slides', metadata,
                         Column('id', Integer, primary_key=True),
                         Column('name', String))

    bullets_table = Table('Bullets', metadata,
                          Column('id', Integer, primary_key=True),
                          Column('slide_id', Integer, ForeignKey('Slides.id')),
                          Column('position', Integer),
                          Column('text', String))

     class Slide(object):
         pass
     class Bullet(object):
         pass

     mapper(Slide, slides_table, properties={
           'bullets': relationship(Bullet, order_by=[bullets_table.c.position])
     })
     mapper(Bullet, bullets_table)

The standard relationship mapping will produce a list-like attribute on each Slide containing all related Bullets, but coping with changes in ordering is totally your responsibility. If you insert a Bullet into that list, there is no magic- it won’t have a position attribute unless you assign it it one, and you’ll need to manually renumber all the subsequent Bullets in the list to accommodate the insert.

An `orderinglist` can automate this and manage the ‘position’ attribute on all related bullets for you.

    mapper(Slide, slides_table, properties={
           'bullets': relationship(Bullet,
                               collection_class=ordering_list('position'),
                               order_by=[bullets_table.c.position])
    })
    mapper(Bullet, bullets_table)

    s = Slide()
    s.bullets.append(Bullet())
    s.bullets.append(Bullet())
    s.bullets[1].position
    >>> 1
    s.bullets.insert(1, Bullet())
    s.bullets[2].position
    >>> 2

Use the `ordering_list` function to set up the `collection_class` on relationships (as in the mapper example above). This implementation depends on the list starting in the proper order, so be SURE to put an order_by on your relationship.

Warning

`ordering_list` only provides limited functionality when a primary key column or unique column is the target of the sort. Since changing the order of entries often means that two rows must trade values, this is not possible when the value is constrained by a primary key or unique constraint, since one of the rows would temporarily have to point to a third available value so that the other row could take its old value. `ordering_list` doesn’t do any of this for you, nor does SQLAlchemy itself.

`ordering_list` takes the name of the related object’s ordering attribute as an argument. By default, the zero-based integer index of the object’s position in the `ordering_list` is synchronized with the ordering attribute: index 0 will get position 0, index 1 position 1, etc. To start numbering at 1 or some other integer, provide `count_from=1`.

Ordering values are not limited to incrementing integers. Almost any scheme can implemented by supplying a custom `ordering_func` that maps a Python list index to any value you require.

## API Reference

`sqlalchemy.ext.orderinglist.``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 relationship’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.

`sqlalchemy.ext.orderinglist.``count_from_0`(index, collection)

  Numbering function: consecutive integers starting at 0.

`sqlalchemy.ext.orderinglist.``count_from_1`(index, collection)

  Numbering function: consecutive integers starting at 1.

`sqlalchemy.ext.orderinglist.``count_from_n_factory`(start)

  Numbering function: consecutive integers starting at arbitrary start.

class `sqlalchemy.ext.orderinglist.``OrderingList`(ordering_attr=None, ordering_func=None, reorder_on_append=False)

  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` relationship definitions.

  `__init__`(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 relationship.

    Parameters:

    - ordering_attr – Name of the attribute that stores the object’s order in the relationship.
    - 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.

  `append`(entity)

    L.append(object) – append object to end

  `insert`(index, entity)

    L.insert(index, object) – insert object before index

  `pop`([index]) → item -- remove and return item at index (default last).

    Raises IndexError if list is empty or index is out of range.

  `remove`(entity)

    L.remove(value) – remove first occurrence of value. Raises ValueError if the value is not present.

  `reorder`()

    Synchronize ordering for the entire collection.

    Sweeps through the list and ensures that each object has accurate ordering information set.