Grails - collection propery don't get rendered to JSON/XML -

i have domain class has collection type property. i'd render json, or xml won't show. don't know why that's so.

here's example:

class bag {      list things  } 

but renders (in json) as:

{"class":"com.mystuff.bag","id":null} 

even if put objects in list. check problems?

update 1

i tried doing same thing on project , can see list getting marshalled json no additional configuration. other project test 1 using more vanilla settings. don't remember changing config on actual project though.

update 2

so testing code , found out json marshalling works fine collections, until place domain class inside @mock, @mock(bag). i'm not sure why happening. assume whatever @mock domain classes, grails run-app well.