Features

django-gm2m…

• 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)
• Does not require you to modify nor monkey-patch the existing model classes that need to be linked
• Provides automatic reverse relations when an instance is added
• Enables related objects prefetching
• Allows the use of Through models
• Allows you to customize the deletion behaviour
• Supports migrations

In this page, we’ll make use of the models that were described in the Quick start section:

>>> from django.db import models
>>> from gm2m import GM2MField
>>>
>>> class Video(models.Model):
>>>     pass
>>>
>>> class Movie(Video):
>>>     pass
>>>
>>> class Documentary(Video):
>>>     pass
>>>
>>> class User(models.Model):
>>>     preferred_videos = GM2MField()
>>>
>>>
>>> me = User.objects.create(name='Me')
>>>
>>> v_for_vendetta = Movie.objects.create(title='V for Vendetta')
>>> citizenfour = Documentary.objects.create(title='Citizenfour')
>>>
>>> me.preferred_videos = [v_for_vendetta, citizenfour]

Reverse relations

We’ve seen how you could access all the preferred_videos of a given user. But what if you want to access all the users that have bookmarked a given video? django-gm2m provides that out of the box, with a bit of magic.

Automatic creation

Indeed, even without having to explicitly create reverse relations (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 after having added movie to a User’s preferred_videos, you can do:

>>> [u.name for u in movie.user_set]
['Me']

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

>>> class Opera(Video):
>>>     pass
>>>
>>> bartered_bride = Opera.objects.create(title="The Bartered Bride")
>>> [u.name for u in bartered_bride.user_set]
AttributeError: 'Opera' object has no attribute 'user_set'

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

Warning

If automatic relations have been added during a session, be aware that they will not necessarily be available in another session. If you restart your server, for example, the automatically created relations will be lost. Read on to find how to tackle this issue.

Manual creation

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 some models to the GM2MField constructor. You may use model names ('app.Model' or 'Model' if you’re in the same module) if necessary to avoid circular references.

Let’s say that instead of:

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

We actually write:

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

Then the reverse relations from Movie and Opera are created when the model classes are created and they are available even if no instance has been added yet:

>>> [u.name for u in bartered_bride.user_set]
[]

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.

Manual creation on existing model

If you need to add relations afterwards, or if the GM2MField is defined in a third-party library you cannot or do not want to patch, you can use the GM2MField’s add_relation method.

Suppose we could not amend our User class to add the reverse relations to Movie and Opera by providing arguments to the GM2MField constructor, this would have exactly the same effect:

>>> User.preferred_videos.add_relation(Movie)
>>> User.preferred_videos.add_relation('Opera')

As shown, you can also use model names (app.Model) with add_relation.

Operations and queries on reverse relations

The reverse relations provide you with the full set of operations that normal Django reverse relation exposes: add, remove and clear. set is also available from version 0.4.2.

A reverse relation also enables you to use lookup chains in your queries:

>>> jack = User.objects.create(name='Jack')
>>> jack.preferred_videos.add(bartered_bride)
>>> [o.name for o in Opera.objects.filter(user__name='Jack')]
['The Bartered Bride']

Related models lookup

From version 0.4.3 onwards, you can access all the models related to a GM2MField using the get_related_models method, that takes an include_auto optional argument if you want to include the automatically created models:

>>> User.preferred_videos.get_related_models()
[<class 'Movie'>, <class 'Opera'>]
>>>
>>> User.preferred_videos.get_related_models(include_auto=True)
[<class 'Movie'>, <class 'Opera'>, <class 'Documentary'>]

Indeed, in that example, Movie, Opera and Theater have been added to preferred_videos, while Documentary has only been automatically added with the addition of citizenfour to me’s preferred videos (at the top of the page).

Deletion

By default, when a source or target model instance is deleted, all relations linking this instance are deleted. It is possible to change this behavior with 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.

Several deletion functions are available:

CASCADE [default]

The relation is deleted with the instance it is related to. The database remains consistent, no ForeignKey nor GenericForeignKey can point to a non-existent object after the operation.

DO_NOTHING

The relation is not deleted with the instance it is related to. It is your responsibility to ensure that the database remains consistent after the deletion operation.

CASCADE_SIGNAL

Same as CASCADE but sends the deleting signal (see Signals below).

CASCADE_SIGNAL_VETO

Sends a deleting signal, and if no receiver vetoes the deletion by returning True or a Truthy value, calls CASCADE. Be careful using this one as when the deletion is vetoed, the database is left in an inconsistent state.

DO_NOTHING_SIGNAL

Same as DO_NOTHING but sends a deleting signal.

Signals

The signals listed below can be imported from the gm2m.signals module.

deleting

Sent when source model (= where the GM2MField is defined) instances are deleted. The sender is the GM2MField instance. The receivers take 2 keyword arguments:

• del_objs, an iterable containing the objects being deleted in the first place
• rel_objs, an iterable containing the objects related to the objects in del_objs, and that are to be deleted if cascade deletion is enabled

This signal can be used to customize the behaviour when deleting a source or target instance.

Prefetching

Prefetching works exactly the same way as with django ManyToManyField:

>>> User.objects.all().prefetch_related('preferred_videos')

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

Through models

Custom 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)

If there is only one ForeignKey to the source model (User in the above example) and only one GenericForeignKey in the target model, they will automatically be used for the relationship. Otherwise, if there are more of them, you must provide a through_fields argument (a list or tuple of 2 to 4 field names) to the GM2MField constructor.

GM2MField constructor’s other parameters

In addition to the specific on_delete* and through / through_fields parameters, you can use the following optional keyword arguments when defining a GM2MField. For the sake of consistency, they have the same signification as in Django’s ManyToManyField and 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 automatically created through 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. Defaults 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 model name.

pk_maxlength

This is useful when using an automatically created intermediate model, to specify the length of the CharField used to store primary keys in the GenericForeignKey. Indeed, the default value of 16 characters may not be sufficient to accomodate certain large foreign key values (e.g. UUIDs). Use None if you don’t want any limitation (this may cause performance issues, though). Defaults to 16.

Migrations

django-gm2m fully supports Django migrations.

When generating migrations for an app using GM2MField, do not be surprised to see a through_fields keyword argument (as a list containing 4 field names) in the migration even if you did not provide it when creating the GM2MField in your model. This is necessary for django’s migrations system to keep track of the arguments assignment and build accurate model representations from the migrations.

System checks

django-gm2m adds a few system checks, derived from built-in django checks for related fields and many to many fields. Here are the errors they may raise, with the builtin counterpart between brackets:

gm2m.E001 [fields.E330]

GM2MFields cannot be unique

gm2m.E101 [fields.E331]

Field specifies a many-to-many relation through model which has not been installed

gm2m.E102 [fields.E333]

The model used as an intermediate model does not have a foreign key to the source model

gm2m.E103 [fields.E334]

The model used as an intermediate model has more than one foreign key to the source model, which is ambiguous (the one that is used is the first declared in the model)

gm2m.E104 [fields.E333]

The model used as an intermediate model does not have a generic foreign key

gm2m.E105 [fields.E334]

The model used as an intermediate model has more than one generic foreign key, which is ambiguous (the one that is used is the first declared in the model).

gm2m.E106 [fields.E337]

The field specifies through_fields but does not provide the names of the two link fields that should be used for the relation through model

gm2m.E107 [fields.E338]

The model used as an intermediate model does not have the field specified in through_fields

gm2m.E108 [fields.E339]

The field specified in through_fields is not a foreign key to the source model

gm2m.E109 [fields.E338]

The model used as an intermediate model does not have the generic foreign key field specified in through_fields

gm2m.E110 [fields.E339]

The field specified in through_fields is not a generic foreign key

gm2m.E201 [fieldsE301]

Field defines a relation with a model that has been swapped out

gm2m.E202 [fields.E302]

Reverse accessor for the field clashes with a field from the target model

gm2m.E203 [fields.E303]

Reverse query name for the field clashes with a field from the target model

gm2m.E204 [fields.E304]

Reverse accessor for the field clashes with reverse accessor from another field

gm2m.E205 [fields.E305]

Reverse accessor for the field clashes with reverse query name from another field

Future improvements

• Add Django admin and possibly limit_choices_to support