Dynamic attribute filtering on serializers

Add a method named filter that by default returns all fields. Actual
serialised implementations can override this method to dynamically
filter the list of attributes to be serialized.

Implements rails-api#1058.

Author: lautis

lib/active_model/serializer.rb

@@ -125,6 +125,10 @@ def json_key
       @root || object.class.model_name.to_s.underscore
     end
 
+    def fields(keys)
+      keys
+    end
+
     def attributes(options = {})
       attributes =
         if options[:fields]
@@ -133,7 +137,7 @@ def attributes(options = {})
           self.class._attributes.dup
         end
 
-      attributes.each_with_object({}) do |name, hash|
+      fields(attributes).each_with_object({}) do |name, hash|
         unless self.class._fragmented
           hash[name] = send(name)
         else

test/serializers/attributes_test.rb

@@ -18,11 +18,24 @@ def test_attributes_definition
           @profile_serializer.class._attributes)
       end
 
+      def test_attributes
+        assert_equal({ name: 'Name 1', description: 'Description 1' },
+          @profile_serializer.attributes)
+      end
+
       def test_attributes_with_fields_option
         assert_equal({ name: 'Name 1' },
           @profile_serializer.attributes(fields: [:name]))
       end
 
+      def test_attributes_with_fields_method
+        @profile_serializer.define_singleton_method(:fields) do |keys|
+          keys - [:name]
+        end
+        assert_equal({ description: 'Description 1' },
+          @profile_serializer.attributes)
+      end
+
       def test_attributes_inheritance_definition
         assert_equal([:id, :body], @serializer_klass._attributes)
       end