You can find some docs here: Ecto.Schema — Ecto v3.7.1

I blogged about a bit of the background to why things are the way they are:
https://kobrakai.de/kolumne/a-case-against-many_to_many/

So the user already exists. So essentially we don’t need to handle the many to many anymore to begin with given you’re not actually affecting both ends. All you insert is the chatroom and the first user of it:

%Chatroom{name: "chatroom_name", chatroom_users: [%{user_id: user.id, role: :owner}]} |> Repo.insert()

One important thing ecto doesn’t do, but people somehow expect is having an API to create relationships to already existing records. Just set the :record_id on wherever you need it. There is Ecto.build_assoc, but usually it doesn’t make things simpler.

It’s totally doable… but not with many_to_many.

With two has_many, and one should be has_many through.

You don’t. Ecto will handle that for you when using the schema/changeset assoc handling. It’ll still be a transaction in the end.