django-positions 0.5.4

Indices

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

The 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 model ordering.

Be aware that, unlike repositioning objects one at a time using list indices, the reposition method will call the save method of every model instance in the queryset.

Many-to-many

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.