Question

I’m having some trouble understanding many-to-many fields in Django.

When I create a many-to-many field, ex:

class GlobalPart (Models.model):
    ...
    category_id=models.ManyToManyField(Category, related_name = 'globalpart')
    ...

and

class Category (Model.model):
    ...
    category = models.CharField(max_length=250)
    ...

I notice that it created a new table called appname_globalpart_category_id in addition to the appname_globalpart table for the GlobalPart model.

What I’m wondering is, how should the field types in that table be defined. I would think that
there should be at least one foreign key there to relate the fields. But instead there is the primary key for the table, and the other fields are integers (globalpart_id and category_id).

So my question is — is that normal? Or did I somehow define the many-to-many field incorrectly? And my next question is how would I get all the category_ids associated to a particular GlobalPart?

Answer 1

(1) short answer: Yes this is normal.

Long answer: ManyToMany table will need a foreign key to both Category and GlobalPart tables. Strictly speaking those two foreign keys should be sufficient. The extra pk that you see in there is just for django magic. You can really get away with only those two foreign keys in that table if you manually define the many-to-many table yourself. However if you let django do it for you (by using ManyToManyField) you get this extra pk

(2) I suggest changing your model fields category_id to categories:

    class GlobalPart (Models.model):
        categories=models.ManyToManyField(Category, related_name = 'globalpart')

This is because, ManyToManyFields refers well to “many” items. This field does not refer to “one” category_id, it refers to all related categories. So when naming it would be natural to name it accordingly.

As for accessing all categories you can do it by accessing the “categories” property. Say if your object instance named global_part, you can access categories like this:

categories = global_part.categories.all()

Instead of all(), you can use filter() or exclude() the same way you use it when querying models.

Here is a link to related django docs