Using result of intermediate insert in transaction with Ecto Multi

Have a look at Ecto.Multi — Ecto v3.14.0

This function receives the result of the previous Multi step as an argument. In this case you’d return the inserted Post from the step and intercept it.

The “whats-new-in-ecto-2.0.1” ebook contains a section doing just this (Composable transactions with Ecto.Multi) (http://pages.plataformatec.com.br/ebook-whats-new-in-ecto-2-0)

@cmkarlsson Thanks, that worked great!

I’ve run into a new issue with Multi.update. When I update a Post’s content, it also adds a Newsfeed entry.

Here is the wrapper function:

  def updateContent(%{id: id, content: content}, _info) do
    post = Repo.get(post, id)
    Content.update_content_and_add_to_newsfeed(post, %{id: id, content: content})
  end

And here is the logic in the Content context:

  def update_content_and_add_to_newsfeed(post, %{id: id, content: content}) do
    multi =
      Multi.new
        |> Multi.update(:post, update_post(post, %{content: content}))
        |> Multi.insert(:newsfeed, %Newsfeed{message: "post updated"})

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

When I run this code, the content updates in the database, but no newsfeed item gets inserted, and I see this error message in the console:

Server: localhost:4000 (http)
Request: POST /graphiql
** (exit) an exception was raised:
    ** (FunctionClauseError) no function clause matching in Ecto.Multi.update/4

Any idea how to fix this?

What does update_post function do? Is it possible that this one updates the database outside of the multi transaction? If so that would explain what you are seeing.

Here it is:

  def update_post(%Post{} = post, attrs) do
    post
    |> Post.changeset(attrs)
    |> Repo.update()
  end

Yes, the Repo.update will update the database outside the transaction. The Multi.update/4 takes a changeset.

You could create a new function:

def post_changeset(%Post{} = post, attrs) do
    post
    |> Post.changeset(attrs)

and use this in the Multi transaction.

Multi.new
|> Multi.update(:post, post_changeset(post, %{content: "Content"})
|> Multi.insert(...)
...

NOTE: Has not been tested

Got it. This is what worked:

|> Multi.update(:post, Post.changeset(post, %{content: content}))

In my case I needed to follow the initial insertion with an insert_all operation. Multi.merge helped.