Restrictions imposed by intermediate many-to-many relationships should be mentioned in many-to-many reference docs

[Daniele Procida]

​https://docs.djangoproject.com/en/dev/topics/db/models/#extra-fields-on-many-to-many-relationships mentions that "you can’t use add, create, or assignment (i.e., beatles.members = [...]) to create relationships" when using intermediate models for many-to-many relationships with through, but ​https://docs.djangoproject.com/en/dev/topics/db/examples/many_to_many/ doesn't, and should.

In the middle of the ​https://docs.djangoproject.com/en/dev/topics/db/models/#intermediary-manytomany section, there are notes about restrictions, and this probably deserves a subheading of its own.

[Tim Graham]

IMO, extra fields on M2M relationships is an advanced topic that doesn't need to be mentioned in /topics/db/examples/many_to_many/ which seems more beginner oriented. It's not clear to me what type note or link or additional example your looking for here.

[Daniele Procida]

​https://docs.djangoproject.com/en/dev/ref/models/relations/ doesn't mention it either.

I think people are likely to land on those pages, and - if they are dealing with intermediate models - may be puzzled to find that add() etc don't work. Only a sentence would be required: "note that if you are using an intermediate model, some of the related manager's methods are disabled. See ​https://docs.djangoproject.com/en/dev/topics/db/models/#extra-fields-on-many-to-many-relationships"

[Tim Graham <timograham@…>]

In a6e3fb80b0e079c49da722515ed71768022264b8:

Fixed #21343 -- Noted many-to-many restrictions with intermediate model.

Thanks EvilDMP for the report.