SQLAlchemy 0.5 Documentation

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Version: 0.5.0beta1 Last Updated: 06/12/08 16:43:00

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 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.

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 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.

def append(self, entity)

L.append(object) -- append object to end

def insert(self, index, entity)

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

def pop(self, index=-1)

L.pop([index]) -> item -- remove and return item at index (default last)

def remove(self, entity)

L.remove(value) -- remove first occurrence of value

def reorder(self)

Synchronize ordering for the entire collection.

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

def __delitem__(self, index)

x.__delitem__(y) <==> del x[y]

def __delslice__(self, start, end)

x.__delslice__(i, j) <==> del x[i:j]

Use of negative indices is not supported.

def __setitem__(self, index, entity)

x.__setitem__(i, y) <==> x[i]=y

def __setslice__(self, start, end, values)

x.__setslice__(i, j, y) <==> x[i:j]=y

Use of negative indices is not supported.

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