Run engine commands through command infrastructure.

railties/lib/rails/command/actions.rb

 module Rails
   module Command
     module Actions
-      private
-        # Change to the application's path if there is no config.ru file in current directory.
-        # This allows us to run `rails server` from other directories, but still get
-        # the main config.ru and properly set the tmp directory.
-        def set_application_directory!
-          Dir.chdir(File.expand_path("../../", APP_PATH)) unless File.exist?(File.expand_path("config.ru"))
+      # Change to the application's path if there is no config.ru file in current directory.
+      # This allows us to run `rails server` from other directories, but still get
+      # the main config.ru and properly set the tmp directory.
+      def set_application_directory!
+        Dir.chdir(File.expand_path("../../", APP_PATH)) unless File.exist?(File.expand_path("config.ru"))
+      end
+
+      if defined?(ENGINE_PATH)
+        def require_application_and_environment!
+          require ENGINE_PATH
         end
 
+        def load_tasks
+          Rake.application.init("rails")
+          Rake.application.load_rakefile
+        end
+
+        def load_generators
+          engine = ::Rails::Engine.find(ENGINE_ROOT)
+          Rails::Generators.namespace = engine.railtie_namespace
+          engine.load_generators
+        end
+      else
         def require_application_and_environment!
           require APP_PATH
           Rails.application.require_environment!
         end
+
+        def load_tasks
+          Rails.application.load_tasks
+        end
+
+        def load_generators
+          Rails.application.load_generators
+        end
+      end
     end
   end
 end

railties/lib/rails/command/base.rb

@@ -15,6 +15,11 @@ class Error < Thor::Error # :nodoc:
       include Actions
 
       class << self
+        # Returns true when the app is a Rails engine.
+        def engine?
+          defined?(ENGINE_ROOT)
+        end
+
         # Tries to get the description from a USAGE file one folder above the command
         # root.
         def desc(usage = nil, description = nil)

railties/lib/rails/commands/help/USAGE

 Usage: bin/rails COMMAND [args] [options]
+<% if engine? %>
+The common Rails commands available for engines are:
+ generate    Generate new code (short-cut alias: "g")
+ destroy     Undo code generated with "generate" (short-cut alias: "d")
+ test        Run tests (short-cut alias: "t")
 
+All commands can be run with -h for more information.
+
+If you want to run any commands that need to be run in context
+of the application, like `bin/rails server` or `bin/rails console`,
+you should do it from the application's directory (typically test/dummy).
+<% else %>
 The most common rails commands are:
  generate    Generate new code (short-cut alias: "g")
  console     Start the Rails console (short-cut alias: "c")
@@ -11,6 +22,6 @@ The most common rails commands are:
              new application called MyApp in "./my_app"
 
 All commands can be run with -h (or --help) for more information.
-
+<% end %>
 In addition to those commands, there are:
 

railties/lib/rails/commands/rake/rake_command.rb

 module Rails
   module Command
     class RakeCommand < Base
+      extend Rails::Command::Actions
+
       namespace "rake"
 
       class << self
@@ -27,12 +29,12 @@ def rake_tasks
             return @rake_tasks if defined?(@rake_tasks)
 
             ActiveSupport::Deprecation.silence do
-              Rails::Command.require_application_and_environment!
+              require_application_and_environment!
             end
 
             Rake::TaskManager.record_task_metadata = true
             Rake.application.instance_variable_set(:@name, "rails")
-            Rails.application.load_tasks
+            load_tasks
             @rake_tasks = Rake.application.tasks.select(&:comment)
           end
 

railties/lib/rails/engine/commands.rb

-require "rails/engine/commands_tasks"
-
-ARGV << "--help" if ARGV.empty?
+require "rails/command"
 
 aliases = {
   "g" => "generate",
@@ -11,4 +9,4 @@
 command = ARGV.shift
 command = aliases[command] || command
 
-Rails::Engine::CommandsTasks.new(ARGV).run_command!(command)
+Rails::Command.invoke command, ARGV

railties/lib/rails/engine/commands_tasks.rb

@@ -26,7 +26,7 @@ def command_whitelist
 
         def help_message
           <<-EOT.strip_heredoc
-            Usage: rails COMMAND [ARGS]
+            Usage: bin/rails COMMAND [ARGS]
 
             The common Rails commands available for engines are:
              generate    Generate new code (short-cut alias: "g")