feat(router): support causation_id: nil to mark events as chain roots

lib/commanded/commands/dispatcher.ex

@@ -158,7 +158,6 @@ defmodule Commanded.Commands.Dispatcher do
   defp to_execution_context(%Pipeline{} = pipeline, %Payload{} = payload) do
     %Pipeline{
       command: command,
-      command_uuid: command_uuid,
       causation_id: causation_id,
       metadata: metadata
     } = pipeline
@@ -175,7 +174,8 @@ defmodule Commanded.Commands.Dispatcher do
 
     %ExecutionContext{
       command: command,
-      causation_id: causation_id || command_uuid,
+      # `causation_id` is resolved at the router boundary (#624)
+      causation_id: causation_id,
       correlation_id: correlation_id,
       metadata: metadata,
       handler: handler_module,

lib/commanded/commands/router.ex

@@ -465,7 +465,12 @@ defmodule Commanded.Commands.Router do
       Options:
 
         - `causation_id` - an optional UUID used to identify the cause of the
-          command being dispatched.
+          command being dispatched. When the option is omitted, the command's
+          own `command_uuid` is used (preserving prior behaviour). Pass
+          `causation_id: nil` explicitly to mark the resulting events as chain
+          roots — they will be persisted with `causation_id` set to `NULL`,
+          letting `WHERE causation_id IS NULL` queries identify roots without
+          a self-join.
 
         - `command_uuid` - an optional UUID used to identify the command being
           dispatched. When omitted, one will be generated automatically.
@@ -568,10 +573,17 @@ defmodule Commanded.Commands.Router do
           opts = Keyword.merge(@command_opts, opts)
 
           application = Keyword.fetch!(opts, :application)
-          causation_id = Keyword.get(opts, :causation_id)
           command_uuid = Router.fetch_command_uuid(opts)
           consistency = Keyword.fetch!(opts, :consistency)
           correlation_id = Keyword.get_lazy(opts, :correlation_id, &UUID.uuid7/0)
+
+          # `Keyword.fetch/2` distinguishes absence from explicit `nil` (#624)
+          causation_id =
+            case Keyword.fetch(opts, :causation_id) do
+              {:ok, value} -> value
+              :error -> command_uuid
+            end
+
           metadata = Keyword.fetch!(opts, :metadata) |> validate_metadata()
 
           retry_attempts = Keyword.get(opts, :retry_attempts)

test/commands/correlation_causation_test.exs

@@ -59,6 +59,16 @@ defmodule Commanded.Commands.CorrelationCasuationTest do
 
       assert event.causation_id == causation_id
     end
+
+    test "should be `nil` on created event when `causation_id: nil` is passed explicitly" do
+      command = %OpenAccount{account_number: "ACC123", initial_balance: 500}
+
+      :ok = BankRouter.dispatch(command, application: BankApp, causation_id: nil)
+
+      [event] = EventStore.stream_forward(BankApp, "ACC123") |> Enum.to_list()
+
+      assert is_nil(event.causation_id)
+    end
   end
 
   describe "`correlation_id`" do