This module provides
PositionField, a model field for
Django_ that allows
instances of a model to be sorted by a user-specified position. Conceptually,
the field works like a list index: when the position of one item is changed, the
positions of other items in the collection are updated in response.
PositionField to your model; that's just about it.
If you want to work with all instances of the model as a single collection,
there's nothing else required. To create collections based on one or more
fields on the model, specify the field names using the
The apps in
positions.examples demonstrate the
In general, the value assigned to a ``PositionField`` will be handled like a list index, to include negative values. Setting the position to ``-2`` will cause the item to be moved to the second position from the end of collection -- unless, of course, the collection has fewer than two elements. Behavior varies from standard list indices when values greater than or less than the maximum or minimum positions are used. In those cases, the value is handled as being the same as the maximum or minimum position, respectively. ``None`` is also a special case that will cause an item to be moved to the last position in its collection. Bulk updates
PositionManager custom manager uses
PositionQuerySet to provide a
reposition method that will update the position of all objects in the
queryset to match the current ordering. If
reposition is called on the
manager itself, all objects will be repositioned according to the default
Be aware that, unlike repositioning objects one at a time using list indices,
reposition method will call the
save method of every model instance
in the queryset.
Specifying a ``ManyToManyField`` as a ``collection`` won't work; use an intermediate model with a ``PositionField`` instead:: class Product(models.Model): name = models.CharField(max_length=50) class Category(models.Model): name = models.CharField(max_length=50) products = models.ManyToManyField(Product, through='ProductCategory', related_name='categories') class ProductCategory(models.Model): product = models.ForeignKey(Product) category = models.ForeignKey(Category) position = PositionField(collection='category') class Meta(object): unique_together = ('product', 'category') Multi-table model inheritance
By default, if a parent model has a position field that declares a collection,
child model instances are ordered independently. This behavior can be changed
by specifying a
parent_link argument identifying the name of the one-to-one
field linking the child model to the parent. If
parent_link is set, all subclass
instances will be part of a single sequence in each collection.
Unique constraints can't be applied to
PositionField because they break
the ability to update other items in a collection all at once. This one was
a bit painful, because setting the constraint is probably the right thing to
do from a database consistency perspective, but the overhead in additional
queries was too much to bear.
After a position has been updated, other members of the collection are updated
using a single SQL
UPDATE statement, this means the
save method of the
other instances won't be called. As a partial work-around to this issue,
auto_now=True will be assigned the current time.