Medium Article: Building Many-to-Many Associations with cast_assoc and Nested Forms in Phoenix and Ecto

registration_controller.ex

defmodule App.RegistrationController do
  # ...
  def new(conn, _params) do
    changeset =
      Account.changeset(
        %Account{memberships: [
          %Membership{user: %User{}}]})
    render conn, "new.html", changeset: changeset
  end
end