Django GenericForeignKey with GenericRelation

#django #orm #python #webdev

Introduction

I hope it's no offense that, Django is a powerful tool for the web. Today I'll talk about GenericForeignKey and GenericRelation. End of the article you will know about GenericForeignKey and GenericRelation.

What is GenericForeignKey?

GenericForeignKey is a foreign key with any model with the help of content type. Still confusing? Let's see some example.

Suppose we have book system where there are some models like following,

class Author(models.Model):
    name = models.CharField(max_length=64)
    # Other fields


class Reader(models.Model):
    name = models.CharField(max_length=64)
    # Other fields


class ActivityLog(models.Model):
    # Here we want to track everything that other users do. So, I'll add GenericForeignKey
    content_type = models.ForeignKey(
        ContentType, default=None, null=True, on_delete=models.SET_NULL, related_name='activity_logs')
    object_id = models.BigIntegerField(default=None, null=True)
    object = GenericForeignKey(ct_field="content_type", fk_field="object_id")
    # Other fields

Here content_type is the FireignKey relation with django content type model. If you want to know more about content type framework of Django then https://docs.djangoproject.com/en/3.0/ref/contrib/contenttypes/#module-django.contrib.contenttypes.
object_id is just a big integer field for holding the related object id.
object is a relation for accessing the generic object. This field will not create column on database. It just model level mapping with the generic object.

What we can do now?

Data access: We can access data using dot[.] operator like,

>>> _log = ActivityLog.objects.first()
>>> _log.object
<Author object>

What we can't do?

We won't query like ActivityLog.objects.filter(object__name__contains='mr x') on the ActivityLog model. But this is not impossible. Here comes GenericRelation.

How to use GenericRelation with GenericForeignkey?

Just need to modify a little bit of our models(those models need to access over queryset.). Let's do it.

class Author(models.Model):
    name = models.CharField(max_length=64)
    # This will not create field on db
    activity_logs = GenericRelation(ActivityLog ,content_type_field='content_type',
        object_id_field='object_id', related_query_name='author')
    # Other fields


class Reader(models.Model):
    name = models.CharField(max_length=64)
    # This will not create field on db
    activity_logs = GenericRelation(ActivityLog ,content_type_field='content_type',
        object_id_field='object_id', related_query_name='reader')
    # Other fields

That's it. We can now write like following...

>>> ActivityLog.objects.filter(author__name__contains='mr x')  # This is for author.
>>> ActivityLog.objects.filter(reader__name__contains='mr x'). # This is for reader.

Hope this helps.