rails 4.0.0.rc1 mountable engine rake db:migrate not work

在rails 3.2 mountable engine中，可以直接使用rake db:migrate，不过在rails 4.0.0.rc1中没用了，可以自己修正下，修改

activerecord-4.0.0.rc1/lib/active_record/railties

  task :load_config do
    ActiveRecord::Base.configurations = ActiveRecord::Tasks::DatabaseTasks.database_configuration || {}
    ActiveRecord::Migrator.migrations_paths = ActiveRecord::Tasks::DatabaseTasks.migrations_paths
  end

修改为：

  task :load_config do
    ActiveRecord::Base.configurations = ActiveRecord::Tasks::DatabaseTasks.database_configuration || {}
    ActiveRecord::Migrator.migrations_paths = ActiveRecord::Tasks::DatabaseTasks.migrations_paths
    if defined?(ENGINE_PATH) && engine = Rails::Engine.find(ENGINE_PATH)
      if engine.paths['db/migrate'].existent
        ActiveRecord::Migrator.migrations_paths += engine.paths['db/migrate'].to_a
      end
    end
  end

如果不修改rails源码，可以在engine的lib/tasks/*.task中添加

namespace :db do
  task :load_config do
    ActiveRecord::Base.configurations = ActiveRecord::Tasks::DatabaseTasks.database_configuration || {}
    ActiveRecord::Migrator.migrations_paths = ActiveRecord::Tasks::DatabaseTasks.migrations_paths
    if defined?(ENGINE_PATH) && engine = Rails::Engine.find(ENGINE_PATH)
      if engine.paths['db/migrate'].existent
        ActiveRecord::Migrator.migrations_paths += engine.paths['db/migrate'].to_a
      end
    end
  end
end