Update Belongs To relationship

post.ex

defmodule CodeCorps.Post do
  use CodeCorps.Web, :model

  alias CodeCorps.MarkdownRenderer

  schema "posts" do
    field :body, :string
    field :markdown, :string
    field :number, :integer
    field :post_type, :string
    field :status, :string, default: "open"
    field :title, :string

    belongs_to :project, CodeCorps.Project
    belongs_to :user, CodeCorps.User

    timestamps()
  end

  def changeset(struct, params \\ %{}) do
    struct
    |> cast(params, [:title, :markdown, :post_type, :status])
    |> validate_required([:title, :markdown, :post_type, :status])
    |> validate_inclusion(:post_type, post_types)
    |> validate_inclusion(:status, statuses)
    |> MarkdownRenderer.render_markdown_to_html(:markdown, :body)
  end

  def create_changeset(struct, params) do
    struct
    |> changeset(params)
    |> cast(params, [:project_id, :user_id])
    |> validate_required([:project_id, :user_id])
    |> assoc_constraint(:project)
    |> assoc_constraint(:user)
  end

  defp post_types do
    ~w{ idea issue task }
  end

  defp statuses do
    ~w{ open closed }
  end
end

post_controller.ex

  def update(conn, %{"id" => id, "data" => data = %{"type" => "post", "attributes" => _post_params}}) do
    post = 
      Post
      |> preload([:project])
      |> Repo.get!(id)
    changeset = Post.create_changeset(post, Params.to_attributes(data))

    case Repo.update(changeset) do
      {:ok, post} ->
        render(conn, "show.json-api", data: post)
      {:error, changeset} ->
        conn
        |> put_status(:unprocessable_entity)
        |> render(CodeCorps.ChangesetView, "error.json-api", changeset: changeset)
    end
  end

post_controller_test.exs

  test "updates related data and renders chosen resource when data is valid", %{conn: conn} do
    user = insert_user()
    project = insert_project()
    project_two = insert_project()
    post = insert_post(%{project_id: project.id, user_id: user.id})
    conn = put conn, post_path(conn, :update, post), %{
      "meta" => %{},
      "data" => %{
        "type" => "post",
        "id" => post.id,
        "attributes" => @valid_attrs,
        "relationships" => relationships(user, project_two)
      }
    }
    data = json_response(conn, 200)["data"]
    
    assert data["type"] == "post"
    assert data["id"] == post.id |> Integer.to_string
    # FAILURE
    assert data["relationships"]["project"]["data"]["id"] == project_two.id |> Integer.to_string
    assert data["relationships"]["project"]["data"]["type"] == "project"
  end