Controller.ex

def show
  case Car.create(conn.assigns.user, params) do
    {:ok, car} ->
      conn
      |> redirect(to: car)
    {:error, changeset} ->
      conn
      |> render("new.html", changeset: changeset)
  end
end

controller_test.exs

defmodule CarControllerTest do
  use MyAppConnCase

  @valid_attrs %{name: "Some Car"}
  @invalid_attrs %{name: ""}

  setup do
    user = User.create(%{name: "Some User})
    {:ok, user: user}
  end

  test "creates resource and redirects when data is valid", %{user: user} do
    conn = post conn(), user_car_path(conn, :create, user), car: @valid_attrs
    assert redirected_to(conn) == car_path(conn, :index)
    car = Repo.get_by(car, @valid_attrs)
    assert car.name == "Some car"
    assert car.user_id == user.id
    assert get_flash(conn, :info) == "Car created successfully."
  end

  test "does not create resource and renders errors when data is invalid", %{user: user} do
    conn = post conn(), user_car_path(conn, :create, user), car: @invalid_attrs
    assert html_response(conn, 200) =~ "New car"
    assert html_response(conn, 200) =~ "Name can't be blank"
  end

  test "redirects when the user does not exist" do
    conn = post conn(), user_car_path(conn, :create, -1), car: @invalid_attrs
    assert redirected_to(conn) == user_path(conn, :index)
    assert get_flash(conn, :error) == "User not found"
  end
 end

Model.ex

defmodule Car do
  ...
  def create(%User{} = user, params) do
    build(user, :cars)
    |> Car.changeset(params)
    |> Repo.insert
  end

end