How to retrieve id from previous inserted table key by Ecto Multi

Question

I would like to retrieve id from previous inserted table primary key by Ecto Multi.

At first, I insert to A main table. then B details table needs A.id. I tried followin

Answer 1

You can do something like the following example which creates a new User record and a new Email record (where the email record is associated to the parent user record via a user_id foreign key).

alias Ecto.Multi

user = get_user_params_from_form() # <-- or where-ever you are getting data
email = get_email_params_from_form()

Multi.new()
    |> Multi.insert(:user, User.changeset(%User{}, user))
    |> Multi.insert(
      :email,
      # Capture the id from the previous operation
      fn %{
           user: %User{
             id: user_id
           }
         } ->
        Email.changeset(%Email{user_id: user_id}, email)
      end
    )

I think this demonstrates the type of relationship you described. Hope it helps!

Answer 2

I think you can use Multi.merge to use the result of the first multi in a second multi.

This is the example from the docs:

multi =
  Ecto.Multi.new()
  |> Ecto.Multi.insert(:post, %Post{title: "first"})

multi
|> Ecto.Multi.merge(fn %{post: post} ->
  # Here, post is the Post inserted in the first multi.
  Ecto.Multi.new()
  |> Ecto.Multi.insert(:comment, Ecto.build_assoc(post, :comments))
end)
|> MyApp.Repo.transaction()

Update: I've added a comment about the post (your A model) that was created in the first multi. You can use post.id and assign it to the related comments (your B model).