Background

I have 3 queries (A, B and C) that need to be run in a transaction. Query A is always run, as well as query B. Query C is run only if query B returns not empty.

These need to be in a transaction, because if query C fails, I want to rollback any changed made by query A.

Code

Currently my code looks like this:

query_A = 
    # some query 

query_B =
     # some other query

query_C =
      # final query

Multi.new()
    |> Multi.update_all(:a, query_A,
      set: [update_value: true]
    )
    |> Multi.all(:all, query_B)
    |> Multi.update_all(:c, query_C,
      set: [update_other_value: true]
    )
    |> Repo.transaction()

Problems

There are several problems here:

1. Multi.all does not have an atom associated, so if this operation fails I don’t know how to identify it
2. I am always running :c. This is not what I want, I only want to do this if the previous Multi.all returns non-empty.

Is it possible to achieve this, using Ecto.Multi? I have seen Multi.run but I am unsure on how to make it fit here.