Merge pull request #4231 from estolfo/MONGOID-3606-foreign-key-poly

MONGOID-3606 Don't evolve foreign keys of polymorphic relations

lib/mongoid/fields/foreign_key.rb

@@ -63,7 +63,11 @@ def foreign_key?
       # @since 3.0.0
       def evolve(object)
         if object_id_field? || object.is_a?(Document)
-          object.__evolve_object_id__
+          if metadata.polymorphic? && constraint
+            constraint.convert(object)
+          else
+            object.__evolve_object_id__
+          end
         else
           related_id_field.evolve(object)
         end

spec/mongoid/fields/foreign_key_spec.rb

@@ -444,6 +444,29 @@
         end
       end
     end
+
+    context "when the metadata is polymoprhic" do
+
+      let(:metadata) do
+        Agent.reflect_on_association(:names)
+      end
+
+      let(:field) do
+        described_class.new(:nameable_id, type: Object, metadata: metadata)
+      end
+
+      let(:value) do
+        BSON::ObjectId.new().to_s
+      end
+
+      let(:evolved) do
+        field.evolve(value)
+      end
+
+      it "does not change the foreign key" do
+        expect(evolved).to be(value)
+      end
+    end
   end
 
   describe "#lazy?" do