orderinglist

author:Jason Kirtland

orderinglist is a helper for mutable ordered relations. It will intercept list operations performed on a relation collection and automatically synchronize changes in list position with an attribute on the related objects. (See Alternate Collection Implementations 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': relation(Bullet, order_by=[bullets_table.c.position])
 })
 mapper(Bullet, bullets_table)

The standard relation 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': relation(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 relations (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 relation.

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. See the [module documentation](rel:docstrings_sqlalchemy.ext.orderinglist) for more information, and also check out the unit tests for examples of stepped numbering, alphabetical and Fibonacci numbering.

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 ...>
>>> mapper(Blurb, blurbs)
<Mapper ...>

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

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 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.