MONGOID-4282 Evolve Strings to BSON::ObjectIds for queries on polymorphic associations

lib/mongoid/association/constrainable.rb

@@ -35,7 +35,11 @@ def convert_to_foreign_key(object)
       private
 
       def convert_polymorphic(object)
-        object.respond_to?(:id) ? object.id : object
+        if object.is_a?(Mongoid::Document)
+          object.id
+        else
+          BSON::ObjectId.mongoize(object)
+        end
       end
     end
   end

spec/mongoid/association/constrainable_spec.rb

@@ -53,5 +53,63 @@
         expect(constrainable.convert_to_foreign_key("testing")).to eq("testing")
       end
     end
+
+    context 'when the association is polymorphic' do
+
+      let(:constrainable) do
+        Post.relations['posteable']
+      end
+
+      let(:result) do
+        constrainable.convert_to_foreign_key(object)
+      end
+
+      context 'when a BSON::ObjectId is passed' do
+
+        let(:object) do
+          BSON::ObjectId.new
+        end
+
+        it 'returns the object id' do
+          expect(result).to eq(object)
+        end
+      end
+
+      context 'when a string is passed' do
+
+        context 'when the string represents an ObjectId' do
+
+          let(:object) do
+            BSON::ObjectId.new.to_s
+          end
+
+          it 'returns the object id' do
+            expect(result).to eq(BSON::ObjectId.from_string(object))
+          end
+        end
+
+        context 'when the string does not represent an ObjectId' do
+
+          let(:object) do
+            'some-other-string'
+          end
+
+          it 'returns the object' do
+            expect(result).to eq(object)
+          end
+        end
+      end
+
+      context 'when a model object is passed' do
+
+        let(:object) do
+          Post.new
+        end
+
+        it 'returns the id' do
+          expect(result).to eq(object.id)
+        end
+      end
+    end
   end
 end

spec/mongoid/fields/foreign_key_spec.rb

@@ -464,7 +464,7 @@
       end
 
       it "does not change the foreign key" do
-        expect(evolved).to be(value)
+        expect(evolved).to eq(BSON::ObjectId.from_string(value))
       end
     end
   end