Working with M2M Intermediary Models

January 03, 2020 . Django

In this article, we will learn how to use through for Django many-to-many relationship. A few times in the last month, I’ve run into the need to add some metadata to a Django many-to-many relationship.

Let’s go through an example the existing models:

Create a model that matches the existing through table and using Meta.db_table:

Now, we change the many-to-many relationship to use the new model:

Now generate a new migration with python manage.py makemigrations. Then we will change it.

The initial migration:

We just need to wrap these operations in migrations.SeparateDatabaseAndState to get the models in sync without screwing up the existing database setup. All of the changes above represent state changes:

Now you can add fields and create additional migrations as normal.

Admin

Admin should look like this:

Filtering on through fields

You should be able to filter on the through table fields by using the lowercase name of the through model.

# querysets
Group.objects.filter(membership__invite_reason="foo")
<QuerySet [<Group: Python>]>

Group.objects.filter(membership__invite_reason="bar")
<QuerySet [<Group: Django>]>

Person.objects.filter(membership__invite_reason="foo")
<QuerySet [<Person: Erdi>]>

Person.objects.filter(membership__invite_reason="bar")
<QuerySet [<Person: Erdi>]>

Membership.objects.filter(invite_reason='foo')
<QuerySet [<Membership: Erdi Python>]>

Membership.objects.filter(invite_reason='bar')
<QuerySet [<Membership: Erdi Django>]>

# related manager on instances
person_instance = Person.objects.first()
person_instance.groups.filter(membership__invite_reason='foo')
<QuerySet [<Group: Python>]>
    
group_instance = Group.objects.first()
group_instance.members.filter(membership__invite_reason='foo')
<QuerySet [<Person: Erdi>]>

In this article you’ve learned how to use a through with many-to-many relationship. If you want to see more information on through, you can visit this.

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