Serialize Discriminator properties as Strings

Previously, Discriminator properties caused problems for ActiveSupport
to_json, as their values are generally DataMapper::Model instances,
which implement as_json as an array of all the models of that type.
This caused an ActiveSupport::JSON::Encoding::CircularReferenceError.

[#1421 state:resolved]

lib/dm-serializer/to_json.rb

@@ -21,7 +21,13 @@ def as_json(options = {})
 
       properties_to_serialize(options).each do |property|
         property_name = property.name
-        result[property_name] = __send__(property_name)
+        value = __send__(property_name)
+        result[property_name] = case property
+                                when DataMapper::Property::Discriminator
+                                  value.to_s
+                                else
+                                  value
+                                end
       end
 
       # add methods

spec/fixtures/vehicle.rb

@@ -0,0 +1,14 @@
+class Vehicle
+  include DataMapper::Resource
+  property :id, Serial
+  property :name, String
+  property :type, Discriminator
+end
+
+class Car < Vehicle
+
+end
+
+class Motorcycle < Vehicle
+
+end

spec/public/to_json_spec.rb

@@ -68,4 +68,8 @@ def deserialize(result)
   it "handles nil for options" do
     expect { Cow.new.as_json(nil) }.to_not raise_error
   end
+
+  it "serializes Discriminator types as strings" do
+    Motorcycle.new.as_json[:type].should == "Motorcycle" 
+  end
 end