django-gm2m 0.2.7

Django generic many-to-many field

© 2014-2015 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 1.8 and matching Python versions (2.6 to 3.4).

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 when an instance is added

• Related objects prefetching

• Through models

• Deletion behaviour customization using signals (Django 1.6+)

• Migrations support

Installation

As straightforward as it can be, using pip:

pip install django-gm2m

You then need to make sure that Django’s contenttype framework is available by checking that django.contrib.contenttypes is mentionned in the INSTALLED_APPS tuple. As django-gm2m itself does not expose any model nor require specific initialisation, there is no need to add gm2m to the INSTALLED_APPS.

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.

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 instances involved in the source side of a GM2M relationship (= instances of the model where the GM2MField is defined) are being 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.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)

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.

Other parameters

In addition to the specific on_delete* and the through / through_fields 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. 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 name of the model.

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). Defaults to 16. Use None if you don’t want any limitation (this may cause performance issues, though).

Migrations

django-gm2m fully supports Django 1.7+ 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 but does not have a foreign key to the source model

gm2m.E103 [fields.E334]

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

gm2m.E105 [fields.E334]

The model used as an intermediate model but 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

• Think about porting the doc to readthedocs as this README is getting a little too long.