Make `.next_migration_number` reusable for third party AR generators.

activerecord/CHANGELOG.md

+*   Make `next_migration_number` accessible for third party generators.
+
+    *Yves Senn*
+
 *   Objects intiantiated using a null relationship will now retain the
     attributes of the where clause.
 

activerecord/lib/rails/generators/active_record.rb

 require 'rails/generators/named_base'
-require 'rails/generators/migration'
 require 'rails/generators/active_model'
+require 'rails/generators/active_record/migration'
 require 'active_record'
 
 module ActiveRecord
   module Generators # :nodoc:
     class Base < Rails::Generators::NamedBase # :nodoc:
-      include Rails::Generators::Migration
+      include ActiveRecord::Generators::Migration
 
       # Set the current directory as base for the inherited generators.
       def self.base_root
         File.dirname(__FILE__)
       end
-
-      # Implement the required interface for Rails::Generators::Migration.
-      def self.next_migration_number(dirname)
-        next_migration_number = current_migration_number(dirname) + 1
-        ActiveRecord::Migration.next_migration_number(next_migration_number)
-      end
     end
   end
 end

activerecord/lib/rails/generators/active_record/migration.rb

+require 'rails/generators/migration'
+
+module ActiveRecord
+  module Generators # :nodoc:
+    module Migration
+      extend ActiveSupport::Concern
+      include Rails::Generators::Migration
+
+      module ClassMethods
+        # Implement the required interface for Rails::Generators::Migration.
+        def next_migration_number(dirname)
+          next_migration_number = current_migration_number(dirname) + 1
+          ActiveRecord::Migration.next_migration_number(next_migration_number)
+        end
+      end
+    end
+  end
+end