Maybe improve Repo.transaction syntax

[Nils]

Hello,

I just wanted to write down some ideas I had today while using Repo.transaction:

At the moment there are two ways to rollback a transaction. Both bring in some overhead, let me show you an example

def save(alice, bob) do

  try do

    Repo.transaction fn ->

      Repo.update!(%{alice | balance: alice.balance - 10})

      Repo.update!(%{bob | balance: bob.balance + 10})

    end

  rescue
    # or some other error...

    error in Ecto.InvalidChangesetError -> {:error, error.changeset}

  end

end

case save(alice, bob) do

  {:ok} -> ...

  {:error, changeset} -> ...

end

In this case I use try/rescue to convert the error into a return type. So that I can use pattern matching like it is done with most elixir apis. But I think try/rescue is quite uncommon so I was looking for another solution.

Another way to rollback the transaction is Repo.rollback and with some elixir 1.3 magic it can look like:

def save(alice, bob) do

  Repo.transaction fn ->

    with {:ok, _} <- Repo.update(%{alice | balance: alice.balance - 10}),

           {:ok, _} <- Repo.update(%{bob | balance: bob.balance + 10}) do 

           

      {:ok}

    else

      {:error, error} -> Repo.rollback(error)

    end

  end

end

Is there another way to do this? I found it quite suprising that Repo.transaction is throwing an exception. I think a better name for it would be Repo.transaction! to be in line with other apis.

So we could have Repo.transaction! throwing the errors from Repo.update! and Repo.rollback. And use Repo.transaction to always return {:ok, _} or {:error, _}

def save(alice, bob) do

  Repo.transaction fn ->
    # Repo.transaction will catch the errors

    Repo.update!(%{alice | balance: alice.balance - 10})

    Repo.update!(%{bob | balance: bob.balance + 10})

  end

end

{:ok, _} = save(alice, bob)

# and

try do

   Repo.transaction! fn ->

     Repo.update!(%{alice | balance: alice.balance - 10})

     Repo.update!(%{bob | balance: bob.balance + 10})

   end

 rescue

   error in Ecto.InvalidChangesetError -> {:error, error.changeset}

 end

This might also be confusing when the Repo.update! errors are not propagated further but catched by Repo.transaction...

Just some ideas, what do you think?

[Gio Torres]

Plus one on this!

I faced the same issue today.

Mostly, I use Ecto.Multi to do transactions, and it has been very satisfactory to me.

Today I had to encapsulate some code on a Repo.transaction/2 without breaking the interface, which was {:ok, result} | {:error, changeset}.

Naively, I did:

Repo.transaction fn ->

    with {:ok, alice} <- Person.add_amount(alice, -10),

           {:ok, bob} <- Person.add_amount(bob, +10) do 

      Ledger.update_balance([alice, bob])

    end

end

where

- Person.add_amount/2 :: {:ok, result} | {:error, reason}

- Ledger.update_balance/2 :: {:ok, result} | {:error, reason}

I was hoping that on Repo.transaction/2, when it receives a tuple {:error, reason}, it would rollback the transaction and return {:error, reason}, but instead it returned {:ok, {:error, reason}}, then I realized I'd have to do like what Nils suggested above:

Repo.transaction fn ->

    with {:ok, alice} <- Person.add_amount(alice, -10),

           {:ok, bob} <- Person.add_amount(bob, +10) do 

      Ledger.update_balance([alice, bob])

    else

      {:error, error} -> Repo.rollback(error)

    end

end

So, I'd suggest that Repo.transaction/2 could consider when the given function returns the conventioned tuples {:ok, result} | {:error, reason} to really bubble up these and commit or rollback based on them. :)

[José Valim]

Unfortunately it is impossible to make this change without breaking backwards compatibility. Although you should be able to build a helper on your own project on top of the current API.

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[Thomas J.]

Does this wrapping code make transaction behave like you'd expect to?
José, even with a new *major* Ecto version it's not possible to change the API ? Instead of raising it would return an error tuple and that will mostly fail the code anyway. Major versions include incompatible API changes.

```elixir
def transaction(fun_or_multi, opts \\ []) do

  try do

    Repo.transaction(fun_or_multi, opts)

  rescue

    error in Ecto.InvalidChangesetError -> {:error, error.changeset}

  end

end

def transaction!(fun_or_multi, opts \\ []) do

  Repo.transaction(fun_or_multi, opts)

end
```