PositionField
**This is a model field for managing user-specified positions.** Usage ===== Add a `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. In order to create collections based on another field in the model (a `ForeignKey`, for example), set `unique_for_field` to the name of the field. It's probably also a good idea to wrap the `save` method of your model in a transaction since it will trigger another query to reorder the other members of the collection. Here's a simple example: from django.db import models, transaction from positions.fields import PositionField class List(models.Model): name = models.CharField(max_length=50) class Item(models.Model): list = models.ForeignKey(List, db_index=True) name = models.CharField(max_length=50) position = PositionField(unique_for_field='list') # not required, but probably a good idea save = transaction.commit_on_success(models.Model.save) 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 the 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. Limitations =========== * 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. More === More information, including an example app and tests, is available on [Google Code](http://code.google.com/p/django-positions/).
- lists
- models
- fields
- model
- field
- list
- sorting
- ordering
- collection
- collections