Repo.transact/2 (The Case Against Ecto.Multi)

After reading Towards Maintainable Elixir by Saša Jurić and hearing about his famous Repo.transact in some of his talks, I decided it was time to explore this for myself.

This post takes into account that you (the reader) are aware and know why and when to use Ecto.Multi. But for those unfamiliar, the TL;DR is you would use an Ecto.Multi when you want to perform multiple transaction that you want to be committed to the database in one shot. Meaning, if one of the transactions fails, you would want to revert all other transactions in the run.

The Problem with Ecto.Multi

Lets get something straight, there is nothing wrong with using Ecto.Multi in your codebase. If it works for you, then it works. However after working with it in multiple codebases I have started to see a common theme: it is very noisy and can be sometimes hard to follow and DRY up. You can get around it by using a lot of private functions to support the Ecto.Multi, but then your module just has a tons of wrapper functions.

What is Repo.transact/2?

The function Repo.transact is our small wrapper around Repo.transaction/2. This function commits the transaction if the lambda returns {:ok, result}, rolling it back if the lambda returns {:error, reason}. In both cases, the function returns the result of the lambda. We chose this approach over Ecto.Multi, because we’ve experimentally established that multi adds a lot of noise with no real benefits for our needs.

– Saša Jurić

Function definition

Saša never gives out the implementation of the function but I came up with this, and it works great; Exactly as you would expect.

defmodule MyApp.Repo do
  use Ecto.Repo,
    otp_app: :my_app,
    adapter: Ecto.Adapters.Postgres

  @doc """
  A small wrapper around `Repo.transaction/2'.

  Commits the transaction if the lambda returns `:ok` or `{:ok, result}`,
  rolling it back if the lambda returns `:error` or `{:error, reason}`. In both
  cases, the function returns the result of the lambda.
  """
  @spec transact((-> any()), keyword()) :: {:ok, any()} | {:error, any()}
  def transact(fun, opts \\ []) do
    transaction(
      fn ->
        case fun.() do
          {:ok, value} -> value
          :ok -> :transaction_commited
          {:error, reason} -> rollback(reason)
          :error -> rollback(:transaction_rollback_error)
        end
      end,
      opts
    )
  end
end

Ecto.Multi vs Repo.transact

Lets say we want to take the typical user registration flow as an example. If we want to insert a user, log the action to an audit table and also enqueue a job to send a confirmation email.

We would have something like this using an Ecto.Multi:

Ecto.Multi implementation

def register_user(params) do
  Multi.new()
  |> Multi.insert(:user, Accounts.new_user_changeset(params))
  |> Multi.insert(:log, fn %{user: user} ->
    Logs.log_action(:user_registered, %{user: user})
  end)
  |> Multi.insert(:email_job, fn %{user: user} ->
    Mailer.enqueue_email_confirmation(user)
  end)
  |> Repo.transaction()
  |> case do
    {:ok, %{user: user}} ->
      {:ok, user}

    {:error, _failed_operation, failed_value, _changes_so_far} ->
      {:error, failed_value}
  end
end

As we can see, it is not the worst, but once we see the Repo.transact/2 way, it will be clear which is better.

Repo.transact implementation

def register_user(params) do
  Repo.transact(fn ->
    with {:ok, user} <- Accounts.create_user(params),
         {:ok, _log} <- Logs.log_action(:user_registered, user),
         {:ok, _job} <- Mailer.enqueue_email_confirmation(user) do
      {:ok, user}
    end
  end)
end

As you can see, it is much shorter and easier to read. Another big benefit is that we do not need to go down to the changeset level for inserting, we could use our functions that perform Repo.inserts in them (Accounts.new_user_changeset/1 vs Accounts.create_user/1). This lets us compose many functions together from outside the context modules without having the need to expose your changeset functions.

The end result is the same, but it is a lot easier to read what is going on IMHO.