Merge pull request mongodb#3533 from rodrigosaito/include_id_in_only

Change in behavior with MongoDB projections and Mongoid 'only' method.

lib/mongoid/attributes.rb

@@ -224,7 +224,7 @@ def write_attributes(attrs = nil)
     alias :attributes= :write_attributes
 
     # Determine if the attribute is missing from the document, due to loading
-    # it from the database with missing fields.
+    # it from the database with missing fields, _id is always loaded
     #
     # @example Is the attribute missing?
     #   document.attribute_missing?("test")
@@ -237,6 +237,7 @@ def write_attributes(attrs = nil)
     def attribute_missing?(name)
       selection = __selected_fields
       return false unless selection
+      selection.merge!({ "_id" => 1 })
       (selection.values.first == 0 && selection[name.to_s] == 0) ||
         (selection.values.first == 1 && !selection.has_key?(name.to_s))
     end

spec/mongoid/criteria_spec.rb

@@ -2640,6 +2640,10 @@ class D
         it "does not add _type to the fields" do
           expect(criteria.options[:fields]["_type"]).to be_nil
         end
+
+        it "always contains the id field" do
+          expect(criteria.first.id).to_not be_nil
+        end
       end
 
       context "when instantiating a class of another type inside the iteration" do