Only add _type for models using inheritance. Closes #2067.

lib/mongoid/criteria.rb

@@ -222,7 +222,7 @@ def extras(extras)
     # @since 2.0.0
     def field_list
       if options[:fields]
-        options[:fields].keys.reject!{ |key| key == "_type" }
+        options[:fields].keys.reject{ |key| key == "_type" }
       else
         []
       end
@@ -438,7 +438,11 @@ def merge!(other)
     # @since 1.0.0
     def only(*args)
       return clone if args.empty?
-      super(*(args + [:_type]))
+      if klass.hereditary?
+        super(*args.push(:_type))
+      else
+        super(*args)
+      end
     end
 
     # Returns true if criteria responds to the given method.

spec/mongoid/criteria_spec.rb

@@ -2746,12 +2746,30 @@
       Band.create(name: "Depeche Mode")
     end
 
-    let(:criteria) do
-      Band.only(:_id)
+    context "when not using inheritance" do
+
+      let(:criteria) do
+        Band.only(:_id)
+      end
+
+      it "limits the returned fields" do
+        criteria.first.name.should be_nil
+      end
+
+      it "does not add _type to the fields" do
+        criteria.options[:fields]["_type"].should be_nil
+      end
     end
 
-    it "limits the returned fields" do
-      criteria.first.name.should be_nil
+    context "when using inheritance" do
+
+      let(:criteria) do
+        Doctor.only(:_id)
+      end
+
+      it "adds _type to the fields" do
+        criteria.options[:fields]["_type"].should eq(1)
+      end
     end
   end