django-gm2m 0.1b1

Django generic many-to-many field

© 2014 Thomas Khyn

Django generic many-to-many field implementation.

This django application exposes a GM2MField that combines the features of the standard Django ManyToManyField and GenericForeighKey and that can be used exactly the same way.

It works with Django 1.4 to 1.7 and matching Python versions (2.6 to 3.4).

Warning

This a beta release. It is unsafe to use a beta release in a production environment.

Features

• Works like the built-in Django related fields

• Creates only one table per relation, like ManyToManyField

• No need to modify the existing model classes that need to be linked

• Reverse relations <model_name>_set

• Related objects prefetching

• Deletion behaviour customization (Django 1.6+)

Installation

As straightforward as it can be, using pip:

pip install django-gm2m

You then need to add gm2m to your INSTALLED_APPS.

You will also need to have django.contrib.contenttypes enabled.

Quick start

You can use the exposed GM2MField exactly the same way as a ManyToManyField.

Suppose you have some models describing videos types:

from django.db import models

class Video(models.Model):
   pass

class Movie(Video):
   pass

class Documentary(Video):
   pass

Now, if you want to have a field for the preferred videos of a User, you simply need to add a default GM2MField to the User model:

class User(models.Model):
   preferred_videos = GM2MField()

Now you can add videos to the preferred_videos set:

user = User.objects.create()
movie = Movie.objects.create()

user.preferred_videos.add(movie)

or:

user.preferred_videos = [movie]

You can obviously mix instances from different models:

documentary = Documentary.objects.create()
user.preferred_videos = [movie, documentary]

Relations

From a User instance, you can now fetch all the user’s preferred videos:

>>> list(user.preferred_videos)
[<Movie object>, <Documentary object>]

However, when using a default GM2MField() (without arguments), it does not create any reverse relations. This means that, from a Movie instance, you cannot retrieve the users who have this instance in their preferred_videos using:

>>> list(movie.user_set)
AttributeError: 'Movie' object has no attribute 'user_set'

To enable this behaviour, you need to let the GM2MField constructor know that you want it to create a reverse relation with given models. To do so, you simply need to provide the models as arguments. You may use model names if necessary to avoid circular imports:

class User(models.Model):
   preferred_videos = GM2MField(Movie, 'Documentary')

Note that this will not prevent the addition of instances from any other models to the generic many-to-many field. This only creates reverse relations.

Deletion

When an instance of an unrelated model is deleted, no relation is deleted, as the unrelated model has no way to know (yet) that one of its instances has been added to a Many-to-Many relation.

By default, when an instance from a related model or source model is deleted, all relations to this instance are deleted. It is possible, from Django 1.6, to change this behavior by using the on_delete, on_delete_src and on_delete_tgt keyword arguments when creating the GM2M field:

from gm2m.deletion import DO_NOTHING

class User(models.Model):
   preferred_videos = GM2MField(Movie, 'Documentary', on_delete=DO_NOTHING)

If you only want this behaviour on one side of the relationship (e.g. on the source model side), use on_delete_src:

class User(models.Model):
   preferred_videos = GM2MField(Movie, 'Documentary',
                                on_delete_src=DO_NOTHING)

on_delete_src and on_delete_tgt override on_delete.

The only customisation is - for the moment - to use the DO_NOTHING function. When using DO_NOTHING, the relation is not deleted with the related instance. It is your responsibility to ensure that the database remains consistent after the deletion operation.

Prefetching

Prefetching works exactly the same way as with django ManyToManyField:

user.objects.prefetch_related('preferred_videos')

will, in a minimum number of queries, prefetch all the videos in all the user’s preferred_video lists.

Through models

Through models are also supported. The minimum requirements for through model classes are:

• one ForeignKey to the source model

• one GenericForeignKey with its ForeignKey and CharField

For example:

class User(models.Model):
   preferred_videos = GM2MField(through='PreferredVideos')

class PreferredVideos(models.Model):
   user = models.ForeignKey(User)
   video = GenericForeignKey(ct_field='video_ct', fk_field='video_fk')
   video_ct = models.ForeignKey(ContentType)
   video_fk = models.CharField(max_length=255)

   ... any relevant field (e.g. date added)

Other parameters

In addition to the specific on_delete* and through parameters, you can use the following optional keyword arguments when defining a GM2MField. Most of them have the same signification than for Django’s ManyToManyField or GenericForeignKey.

verbose_name

A human-readable name for the field. Defaults to a munged version of the model class name.

db_table

The name of the database table to use for the model. Defaults to '<app_label>_<model_name>'.

db_constraint

Controls whether or not a constraint should be created in the database for the internal foreign keys when the through model is automatically created. Defaults to True.

for_concrete_model

If set to False, the field will be able to reference proxy models. Default to True.

related_name

The name that will be used for the relation from a related object back to this one. The same related name is used for all the related models. Defaults to '<src_model_name>_set'.

related_query_name

The name to use for the reverse filter name from the target model. Defaults to the value of related_name or the name of the model.

Future improvements

• Automatic creation of relations when an instance of an unrelated model is added to the many-to-many. This implies deletion as well.

• More deletion behavior options (possibility to pass any custom function?)

• Add Django admin and possibly limit_choices_to support