gistfile1.txt

  def create_section(game, attrs \\ %{}) do
    section = Ecto.build_assoc(game, :sections)
    IO.puts("SECTION: #{inspect(section, pretty: true)}")

    section = Proj23.Level.Section.changeset(section, attrs)
    IO.puts("SECTION: #{inspect(section, pretty: true)}")
  end

RESULTS

Params are: %{"title" => "some title"}
SECTION: %Proj23.Level.Section{
  __meta__: #Ecto.Schema.Metadata<:built, "sections">,
  game: #Ecto.Association.NotLoaded<association :game is not loaded>,
  game_id: 57,
  id: nil,
  inserted_at: nil,
  title: nil,
  updated_at: nil
}
SECTION: #Ecto.Changeset<
  action: nil,
  changes: %{title: "some title"},
  errors: [game: {"can't be blank", [validation: :required]}],
  data: #Proj23.Level.Section<>,
  valid?: false
>



CHANGESET

defmodule Proj23.Level.Section do
  use Ecto.Schema
  import Ecto.Changeset


  schema "sections" do
    field :title, :string

    belongs_to :game, Proj23.Level.Game

    timestamps()
  end

  @doc false
  def changeset(section, attrs) do
    section
    |> cast(attrs, [:title])
    |> cast_assoc(:game)
    |> validate_required([:title, :game])
  end
end