Elixir Ecto cast_assoc

cast_assoc.ex

defmodule User do
  def changeset(user, params) do
    user
    |> cast(params, @allowed_params)
    |> validate_required(@required_params)
    # Account.changeset is inferred by default
    |> cast_assoc(:account, required: true, with: &Account.registration_changeset/2)
  end
end

account = %{plan: "free"}
user = %{email: "coolio@example.com", password: "bananas", account: account}
User.changeset(%User{}, user)