You can do:

def get_setting_for_forum(%Forum{} = forum) do
  caller = self()
  Cachex.fetch!(:cforum, "settings/forums/#{forum.forum_id}", fn ->
    from(setting in Setting, where: setting.forum_id == ^forum.forum_id)
    |> Repo.one(caller: caller)
  end)
end

https://hexdocs.pm/ecto_sql/Ecto.Adapters.SQL.Sandbox.html#module-collaborating-processes

One gotcha with this - AFAIK there’s no way to “un-allow” a connection, and Ecto gets grumpy if you call allow with a second PID. This makes for intermittently-failing tests when there’s a worker pool, as you’ll only get an error when a worker gets reused.

We ran into this issue with :poolboy; our fix was to have workers return :stop in tests so they don’t get re-used. This works, EXCEPT for very occasionally when there’s already a process waiting for a pool worker when the call returns; a fast-path optimization in Poolboy hands the now-exiting worker PID to the waiting process, which then fails

Thanks @benwilson512, but I already use :shared mode on the connection, so it shouldn’t make a difference. (I also tried and it didn’t).

I don’t get the cannot find ownership process error, which would be solved with allow/3 or :shared, but a connection not available.

For now, I “solved” the issue by replacing the db-call inside the existing transaction with a mock that does not hit the db.

Here is a small example that reproduces my issue:

test "calling db from another process inside a transaction" do
  Repo.transaction(fn ->
    parent = self()

    spawn_link(fn ->
      User |> Repo.one()
      send(parent, :done_in_other_process)
    end)

    receive do
      :done_in_other_process -> :ok
    end

    :ok
  end)
end

This is the setup code that is run before each db test:

:ok = Ecto.Adapters.SQL.Sandbox.checkout(MyApp.Repo)
Ecto.Adapters.SQL.Sandbox.mode(MyApp.Repo, {:shared, self()})

It will give this error:

  1) test calling db from another process inside a transaction (MyApp.MyTest)
     test/my_test.exs:153
     ** (EXIT from #PID<0.497.0>) an exception was raised:
         ** (DBConnection.ConnectionError) connection not available and request was dropped from queue after 989ms. You can configure how long requests wait in the queue using :queue_target and :queue_interval. See DBConnection.start_link/2 for more information
             (ecto_sql) lib/ecto/adapters/sql.ex:605: Ecto.Adapters.SQL.raise_sql_call_error/1
             (ecto_sql) lib/ecto/adapters/sql.ex:538: Ecto.Adapters.SQL.execute/5
             (ecto) lib/ecto/repo/queryable.ex:153: Ecto.Repo.Queryable.execute/4
             (ecto) lib/ecto/repo/queryable.ex:18: Ecto.Repo.Queryable.all/3
             (ecto) lib/ecto/repo/queryable.ex:67: Ecto.Repo.Queryable.one/3
             test/my_test.exs:158: anonymous fn/1 in MyApp.MyTest."test calling db from another process inside a transaction"/1