Prefer custom #to_json implementations over #serializable_hash

lib/grape/middleware/base.rb

@@ -111,6 +111,11 @@ def decode_json(object)
           MultiJson.load(object)
         end
 
+        def defines_own_to_json?(object)
+          return false unless object.respond_to?(:to_json)
+          object.method(:to_json).owner.name =~ /^JSON::Ext::Generator::GeneratorMethods/ ? false : true
+        end
+
         def serializable?(object)
          object.respond_to?(:serializable_hash) ||
            object.kind_of?(Array) && !object.map {|o| o.respond_to? :serializable_hash }.include?(false) ||
@@ -131,6 +136,7 @@ def serialize(object)
 
         def encode_json(object)
           return object if object.is_a?(String)
+          return object.to_json if defines_own_to_json?(object)
           return MultiJson.dump(serialize(object)) if serializable?(object)
           return object.to_json if object.respond_to?(:to_json)
 

spec/grape/middleware/formatter_spec.rb

@@ -24,6 +24,22 @@ def to_json
       subject.call({'PATH_INFO' => '/somewhere', 'HTTP_ACCEPT' => 'application/json'}).last.each{|b| b.should == '"bar"'}
     end
 
+    it 'should call #to_json if custom defined instead of #serializable_hash' do
+      class SimpleJsonExample
+        def serializable_hash
+          {:abc => 'def'}
+        end
+
+        def to_json
+          '{"123":"456"}'
+        end
+      end
+
+      @body = SimpleJsonExample.new
+
+      subject.call({'PATH_INFO' => '/somewhere', 'HTTP_ACCEPT' => 'application/json'}).last.each{|b| b.should == '{"123":"456"}'}
+    end
+
     it 'should serialize the #serializable_hash if that is available' do
       class SimpleExample
         def serializable_hash