Use an UPSERT SQL query to insert or update the projection version

lib/projections/ecto.ex

@@ -59,37 +59,40 @@ defmodule Commanded.Projections.Ecto do
         projection_name = Map.fetch!(metadata, :handler_name)
         event_number = Map.fetch!(metadata, :event_number)
 
-        changeset =
-          %ProjectionVersion{projection_name: projection_name}
-          |> ProjectionVersion.changeset(%{last_seen_event_number: event_number})
+        projection_version = %ProjectionVersion{
+          projection_name: projection_name,
+          last_seen_event_number: event_number
+        }
 
         prefix = schema_prefix(event, metadata)
 
+        # Query to update an existing projection version with the last seen event number with
+        # a check to ensure that the event has not already been projected.
+        update_projection_version =
+          from(pv in ProjectionVersion,
+            where:
+              pv.projection_name == ^projection_name and pv.last_seen_event_number < ^event_number,
+            update: [set: [last_seen_event_number: ^event_number]]
+          )
+
         multi =
           Ecto.Multi.new()
-          |> Ecto.Multi.run(:verify_projection_version, fn repo, _changes ->
-            version =
-              case repo.get(ProjectionVersion, projection_name, prefix: prefix) do
-                nil ->
-                  repo.insert!(
-                    %ProjectionVersion{
-                      projection_name: projection_name,
-                      last_seen_event_number: 0
-                    },
-                    prefix: prefix
-                  )
-
-                version ->
-                  version
-              end
-
-            if version.last_seen_event_number < event_number do
-              {:ok, %{version: version}}
-            else
-              {:error, :already_seen_event}
+          |> Ecto.Multi.run(:track_projection_version, fn repo, _changes ->
+            try do
+              repo.insert(projection_version,
+                prefix: prefix,
+                on_conflict: update_projection_version,
+                conflict_target: [:projection_name]
+              )
+            rescue
+              exception in Ecto.StaleEntryError ->
+                # Attempted to insert a projection version for an already seen event
+                {:error, :already_seen_event}
+
+              exception ->
+                reraise exception, __STACKTRACE__
             end
           end)
-          |> Ecto.Multi.update(:projection_version, changeset, prefix: prefix)
 
         with %Ecto.Multi{} = multi <- apply(multi_fn, [multi]),
              {:ok, changes} <- transaction(multi) do
@@ -99,7 +102,7 @@ defmodule Commanded.Projections.Ecto do
             :ok
           end
         else
-          {:error, :verify_projection_version, :already_seen_event, _changes} -> :ok
+          {:error, :track_projection_version, :already_seen_event, _changes} -> :ok
           {:error, _stage, error, _changes} -> {:error, error}
           {:error, _error} = reply -> reply
         end

test/projections/ecto_projection_test.exs

@@ -68,6 +68,27 @@ defmodule Commanded.Projections.EctoProjectionTest do
     assert_seen_event("Projector", 3)
   end
 
+  test "should prevent an event being projected more than once" do
+    Projector.handle(%AnEvent{name: "Event1"}, %{handler_name: "Projector", event_number: 1})
+    Projector.handle(%AnEvent{name: "Event2"}, %{handler_name: "Projector", event_number: 2})
+
+    tasks =
+      Enum.map(1..3, fn _index ->
+        Task.async(fn ->
+          :timer.sleep(:rand.uniform(10))
+
+          Projector.handle(%AnEvent{name: "Event3"}, %{handler_name: "Projector", event_number: 3})
+        end)
+      end)
+
+    results = Task.await_many(tasks)
+
+    assert Enum.uniq(results) == [:ok]
+
+    assert_projections(Projection, ["Event1", "Event2", "Event3"])
+    assert_seen_event("Projector", 3)
+  end
+
   test "should return an error on failure" do
     assert {:error, :failure} ==
              Projector.handle(%ErrorEvent{}, %{handler_name: "Projector", event_number: 1})