Repository: slashdotdash/commanded (elixir)

Issue:
Validate command dispatch registration in router
When using the `Commanded.Commands.Router` to configure command dispatch, it would be helpful to validate that the command, handler, and aggregate modules exist.

In the following example, if the two module `alias` statements are forgotten then command dispatch will fail (`UndefinedFunctionError` module is not available).
```elixir
defmodule MyApp.BankRouter do
  use Commanded.Commands.Router

  alias MyApp.OpenAccount
  alias MyApp.BankAccount

  dispatch OpenAccount, to: BankAccount, identity: :account_number
end
```

Instead, this should cause a compilation failure, prompting the user that the modules cannot be found.

Pull request description:
 Validate command dispatch registration in router
Ensure the command, handler, and aggregate modules exist when configured in a router module.

Fixes #59.

Patch:
diff --git a/.travis.yml b/.travis.yml
index 01959fd..82faecc 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,7 +1,7 @@
 language: elixir
 
 elixir:
-  - 1.5.0
+  - 1.5.1
 
 otp_release:
   - 20.0
diff --git a/CHANGELOG.md b/CHANGELOG.md
index fbf4c31..374ad1a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,9 +1,12 @@
 # Changelog
 
-## v0.14.0 (unreleased)
+## v0.14.0
+
+### Enhancements
 
 - Dispatch command with `:eventual` or `:strong` consistency guarantee ([#82](https://github.com/slashdotdash/commanded/issues/82)).
 - Include custom metadata during command dispatch ([#61](https://github.com/slashdotdash/commanded/issues/61)).
+- Validate command dispatch registration in router ([59](https://github.com/slashdotdash/commanded/issues/59)).
 
 ## v0.13.0
 
diff --git a/lib/commanded/commands/router.ex b/lib/commanded/commands/router.ex
index 70af52a..5bcac50 100644
--- a/lib/commanded/commands/router.ex
+++ b/lib/commanded/commands/router.ex
@@ -71,6 +71,7 @@ defmodule Commanded.Commands.Router do
       :ok = BankRouter.dispatch(command, metadata: %{"ip_address" => "127.0.0.1"})
 
   """
+
   defmacro __using__(_) do
     quote do
       require Logger
@@ -136,11 +137,16 @@ defmodule Commanded.Commands.Router do
 
   @doc false
   defmacro register(command_module, to: handler, function: function, aggregate: aggregate, identity: identity, timeout: timeout, lifespan: lifespan, consistency: consistency) do
-    quote do
+    quote location: :keep do
       if Enum.member?(@registered_commands, unquote(command_module)) do
         raise "duplicate command registration for: #{inspect unquote(command_module)}"
       end
 
+      # sanity check the configured modules exist
+      ensure_module_exists(unquote(command_module))
+      ensure_module_exists(unquote(handler))
+      ensure_module_exists(unquote(aggregate))
+
       handler_functions = unquote(handler).__info__(:functions)
       unless Keyword.get(handler_functions, unquote(function)) == 2 do
         raise "command handler #{inspect unquote(handler)} does not define a function: #{unquote(function)}/2"
@@ -230,6 +236,13 @@ defmodule Commanded.Commands.Router do
     end
   end
 
+  @doc false
+  def ensure_module_exists(module) do
+    unless Code.ensure_compiled?(module) do
+      raise "module `#{inspect module}` does not exist, perhaps you forgot to `alias` the namespace"
+    end
+  end
+
   defp parse_opts([{:to, aggregate_or_handler} | opts], result) do
     case Keyword.pop(opts, :aggregate) do
       {nil, opts} ->


Test patch:
diff --git a/test/commands/routing_commands_test.exs b/test/commands/routing_commands_test.exs
index d75f712..5b089bd 100644
--- a/test/commands/routing_commands_test.exs
+++ b/test/commands/routing_commands_test.exs
@@ -112,6 +112,51 @@ defmodule Commanded.Commands.RoutingCommandsTest do
     end
   end
 
+  test "should prevent registrations for a invalid command module" do
+    assert_raise RuntimeError, "module `UnknownCommand` does not exist, perhaps you forgot to `alias` the namespace", fn ->
+      Code.eval_string """
+        alias Commanded.ExampleDomain.BankAccount
+        alias Commanded.ExampleDomain.OpenAccountHandler
+
+        defmodule InvalidCommandRouter do
+          use Commanded.Commands.Router
+
+          dispatch UnknownCommand, to: OpenAccountHandler, aggregate: BankAccount, identity: :account_number
+        end
+      """
+    end
+  end
+
+  test "should prevent registrations for an invalid command handler module" do
+    assert_raise RuntimeError, "module `UnknownHandler` does not exist, perhaps you forgot to `alias` the namespace", fn ->
+      Code.eval_string """
+        alias Commanded.ExampleDomain.BankAccount
+        alias Commanded.ExampleDomain.BankAccount.Commands.OpenAccount
+
+        defmodule InvalidHandlerRouter do
+          use Commanded.Commands.Router
+
+          dispatch OpenAccount, to: UnknownHandler, aggregate: BankAccount, identity: :account_number
+        end
+      """
+    end
+  end
+
+  test "should prevent registrations for an invalid aggregate module" do
+    assert_raise RuntimeError, "module `UnknownAggregate` does not exist, perhaps you forgot to `alias` the namespace", fn ->
+      Code.eval_string """
+        alias Commanded.ExampleDomain.BankAccount.Commands.OpenAccount
+        alias Commanded.ExampleDomain.OpenAccountHandler
+
+        defmodule InvalidAggregateRouter do
+          use Commanded.Commands.Router
+
+          dispatch OpenAccount, to: OpenAccountHandler, aggregate: UnknownAggregate, identity: :account_number
+        end
+      """
+    end
+  end
+
   defmodule MultiCommandRouter do
     use Commanded.Commands.Router