Added MigrationProxy to defer loading of Migration classes until they are actually required by the migrator

Signed-off-by: Michael Koziarski <michael@koziarski.com>
[#747 state:resolved]

activerecord/lib/active_record/migration.rb

@@ -349,6 +349,27 @@ module ActiveRecord
     end
   end
 
+  # MigrationProxy is used to defer loading of the actual migration classes
+  # until they are needed
+  class MigrationProxy
+
+    attr_accessor :name, :version, :filename
+
+    delegate :migrate, :announce, :write, :to=>:migration
+
+    private
+
+      def migration
+        @migration ||= load_migration
+      end
+
+      def load_migration
+        load(filename)
+        name.constantize
+      end
+
+  end
+
   class Migrator#:nodoc:
     class << self
       def migrate(migrations_path, target_version = nil)
@@ -437,7 +458,7 @@ module ActiveRecord
       runnable.pop if down? && !target.nil?
       
       runnable.each do |migration|
-        Base.logger.info "Migrating to #{migration} (#{migration.version})"
+        Base.logger.info "Migrating to #{migration.name} (#{migration.version})"
 
         # On our way up, we skip migrating the ones we've already migrated
         # On our way down, we skip reverting the ones we've never migrated
@@ -470,11 +491,10 @@ module ActiveRecord
             raise DuplicateMigrationNameError.new(name.camelize) 
           end
           
-          load(file)
-          
-          klasses << returning(name.camelize.constantize) do |klass|
-            class << klass; attr_accessor :version end
-            klass.version = version
+          klasses << returning(MigrationProxy.new) do |migration|
+            migration.name     = name.camelize
+            migration.version  = version
+            migration.filename = file
           end
         end
         

activerecord/test/cases/migration_test.rb

@@ -922,6 +922,26 @@ if ActiveRecord::Base.connection.supports_migrations?
       migrations[0].name    == 'innocent_jointable'
     end
 
+    def test_only_loads_pending_migrations
+      # migrate up to 1
+      ActiveRecord::Migrator.up(MIGRATIONS_ROOT + "/valid", 1)
+
+      # now unload the migrations that have been defined
+      PeopleHaveLastNames.unloadable
+      ActiveSupport::Dependencies.remove_unloadable_constants!
+
+      ActiveRecord::Migrator.migrate(MIGRATIONS_ROOT + "/valid", nil)
+
+      assert !defined? PeopleHaveLastNames
+
+      %w(WeNeedReminders, InnocentJointable).each do |migration|
+        assert defined? migration
+      end
+
+    ensure
+      load(MIGRATIONS_ROOT + "/valid/1_people_have_last_names.rb")
+    end
+
     def test_migrator_interleaved_migrations
       ActiveRecord::Migrator.up(MIGRATIONS_ROOT + "/interleaved/pass_1")