django-gm2m 0.1b4

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. But feel free to experiment with it and report the bugs you may find!

Features

• Works like the built-in Django related fields

• Creates one table per relation, like ManyToManyField, and not one big table linking anything to anything (django-generic-m2m’s default approach)

• No need to modify nor monkey-patch the existing model classes that need to be linked

• Automatic reverse relations <model_name>_set when an instance is added

• Related objects prefetching

• Through models

• 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 obviously 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:

from gm2m import GM2MField

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>]

Note: yes, the >>> are misplaced. This is voluntary. >>> indicates an output value rather than a console input, for the sake of readability.

The magic here is that, even without having to explicitly create reverse relation (e.g by providing models to the GM2MField constructor), they are automatically created when an instance of a yet unknown model is added. This means that you can do:

list(movie.user_set)
>>> [<User object>]

However, it is important to remember that if no instance of a model as ever been added to the set, retrieving the <modelname_set> will raise an AttributeError:

class Opera(Video):
    pass
opera = Opera.objects.create()
list(opera.user_set)
>>> AttributeError: 'Opera' object has no attribute 'user_set'
user.preferred_videos.add(opera)
list(opera.user_set)
>>> [<User object>]

Indeed, the GM2MField has no idea what relation it is expected to create until you provide it with a minimum of information.

However, if you want some reverse relations to be created before any instance is added, so that retrieving the <modelname_set> attribute never raises an exception, it is possible to explicitly provide a list of models as arguments of the GM2MField constructor. You may use model names if necessary to avoid circular imports:

class Concert(Video):
    pass

class User(models.Model):
   preferred_shows = GM2MField('Opera', Concert)

This way, the reverse relations are created when the model class is created and available even if no instance has been added yet:

concert = Concert.objects.create()
list(concert.user_set)
>>> []

If you need to add relations afterwards, or if the GM2MField is defined in a third-party library you do not want to patch, you can still manually add relations afterwards:

class Theater(Video):
   pass
User.preferred_shows.add_relation(Theater)

Note that providing models to GM2MField does not prevent you from adding instances from other models.You can still add instances from other models, and the relation will be created. Providing a list of models will only create reverse relations by default, nothing more.

The reverse relations provide you with the full set of operations that normal Django reverse relation exposes: add, remove and clear.

Deletion

By default, when an instance from a source or target model is deleted, all relations linking this instance are deleted. It is possible, if you are using Django 1.6 or later, to change this behavior by using the on_delete, on_delete_src and on_delete_tgt keyword arguments when creating the GM2MField:

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 or on_delete_tgt:

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 available 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

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

• Add Django admin and possibly limit_choices_to support