Django Generic many to many field implementation

Django application has a GM2MField that combines the features of the standard Django "ManyToManyField" and "GenericForeignKey".

Installation:

pip install django-gm2m

Django’s contenttype framework must be (django.contrib.contenttypes) mentionned in the INSTALLED_APPS.

INSTALLED_APPS = [
   ...
   'django.contrib.contenttypes',
   ...
   'gm2m',
]

Uses Of django-gm2m:
1. reverse relations
2. prefetching
3. Allows you to customize the deletion behaviour

Example Models:

from django.db import models

class Videos(models.Model):
      name = models.CharField(maxlength=100)

class Articles(models.Model):
      name = models.CharField(maxlength=100)

If you want to have a field for the preferred videos and articles of a User, you need to add GM2MField to the User model.

from gm2m import GM2MField

class User(models.Model):
      email = models.CharField(maxlength=100)
      username = models.CharField(maxlength=100)
      related = models.GM2MField()

Now you can add videos, articles to the related set.
Creating User object:

user = User.objects.create(email="mp@mp.com", username="mp@mp.com")

video = Videos.objects.create(name="video")

article = Articles.objects.create(name="sample document")

user.related.add(video)

user.related.add(article)

or you can add both videos, articles instances.

user.related = [video, article]

From User instance, you can fetch all related videos, articles set or you can filter model using the "Model" or "Model__in" keywords.

list(user.related) # This provides list of model objects

list(user.related.filter(Model=Videos)) # This provides list of user related video instances

list(user.related.filter(Model__in=[Videos, Articles]))

1. Reverse Relations:

GM2MField provides automatic reverse relations when an instance is added.

We have seen how to access videos, articles of a given user.

Here we see how to access list of all the users with the given Video or Article instance.

list(video.user_set) # This provides list of user instances

With the user reverse relation, we can also use lookup chains in your queries, can also perform add, remove, clear, set operations

[each.email for each in Video.objects.filter(user__email='mp@mp.com')]

We can also list the models related to GM2MField using get_related_models method

    User.related.get_related_models()

this will give article, video objects as they are already added to user using GM2MField

2. Prefetching

GM2MField provides prefetching in the same way with ManyToManyField

    User.objects.all().prefetch_related('related')

3. Allows you to customize the deletion behaviour

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 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)

But be ensure that the database remains consistent after the deletion operation.

Posted On 16 October 2012 By MicroPyramid

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