Using Ecto v2.2.6, Phoenix 1.3

I have a blog app. When a user makes a Post, it inserts into the Post table, then it get the resulting id of that post and inserts that into a Newsfeeditem table. Ideally, I would like for this to happen as a transaction.

(I am using Absinthe graphql, so my return for the insert must be of the form {:ok, post})

I have a working function that looks like this:

  def create_post_add_newsfeed(%{
      title: title,
      content: content,
      user_id: user_id
    }) do

    case Repo.insert!(%Post{title: title, content: content, user_id: user_id}) do
      post ->
        case Repo.insert!(%Newsfeeditem{type: "user_creates_post", user_id: user_id, post_id: post.id}) do
          newsfeeditem ->
            {:ok, post}
          _ ->
            {:error, "Post not recorded in newsfeed"}
        end
      _ ->
      {:error, "Post not inserted"}
    end
  end

This code is not a transaction, and it reeks of callback stink. Ecto.Multi seems like a more appropriate tool to use here, but I do not know how to get the result of the Post insert so that I can insert it into Newsfeed.

I would like to do something like this

  def create_post_add_newsfeed(%{
      title: title,
      content: content,
      user_id: user_id
    }) do
    multi =
      Multi.new
        |> Multi.insert(:post, %Post{title: title, content: content, user_id: user_id})
        |> # Some intermediate step where I get the 'post' from the line above
        |> Multi.insert(:newsfeeditem, %Newsfeeditem{type: "user_creates_post", user_id: users_id, post_id: post.id})

    case Repo.transaction(multi) do
      {:ok, %{post: post}} ->
        {:ok, post}
      {:error, _} ->
        {:error, "Error"}
    end
  end

Any idea how to pull that off?