Medium Article: Building Many-To-Many Associations with Embedded Schemas in Ecto and Phoenix

registration.ex

defmodule App.Registration do
  use App.Web, :model
  alias App.{Account, User, Membership, Repo}

  embedded_schema do
    field :email
    field :org_name
  end

  @required_fields ~w(email org_name)a

  def changeset(struct, params \\ %{}) do
    struct
    |> cast(params, @required_fields)
    |> validate_required(@required_fields)
  end

  def to_multi(params \\ %{}) do
    Ecto.Multi.new
      |> Ecto.Multi.insert(:account, account_changeset(params))
      |> Ecto.Multi.insert(:user, user_changeset(params))
      |> Ecto.Multi.run :membership, fn changes ->
        Repo.insert membership_changeset(changes)
      end
  end

  defp account_changeset(%{"org_name" => org_name}) do
    Account.changeset %Account{name: org_name}
  end

  defp user_changeset(params) do
    user_params = Map.take(params, ["email"]) # ... name, password, etc...
    User.changeset %User{}, user_params
  end

  defp membership_changeset(changes) do
    %Membership{account_id: changes.account.id,
      user_id: changes.user.id,
      role: :admin}
  end
end