[core] simplify Transformer setup

core/lib/rom/transformer.rb

@@ -15,6 +15,31 @@ class Transformer < Transproc::Transformer[ROM::Processor::Transproc::Functions]
 
     defines :relation, :register_as
 
+    # Configure relation for the transformer
+    #
+    # @example with a custom name
+    #   class UsersMapper < ROM::Transformer
+    #     relation :users, as: :json_serializer
+    #
+    #     map do
+    #       rename_keys user_id: :id
+    #       deep_stringify_keys
+    #     end
+    #   end
+    #
+    #   users.map_with(:json_serializer)
+    #
+    # @param name [Symbol]
+    # @param options [Hash]
+    # @option options :as [Symbol] Mapper identifier
+    #
+    # @api public
+    def self.relation(name = Undefined, options = EMPTY_HASH)
+      return @relation if name.equal?(Undefined) && defined?(@relation)
+      register_as(options.fetch(:as, name))
+      super(name)
+    end
+
     # Define transformation pipeline
     #
     # @example

core/spec/integration/transformer_spec.rb

@@ -20,8 +20,7 @@
 
   let(:mapper_class) do
     Class.new(ROM::Transformer) do
-      relation :users
-      register_as :default
+      relation :users, as: :default
 
       map do
         rename_keys user_id: :id