[{"sections":[{"name":"section 1","questions":[{"q":"color","a":"black"},{"q":"size","a":"large"}]}]},{"sections":[{"name":"section 1","questions":[{"q":"color","a":"white"},{"q":"size","a":"large"}]},{"name":"section 2","questions":[{"q":"color","a":"black"},{"q":"size","a":"small"}]}]}]

db.collection.find({$and:[{"sections.questions":{"q":"color","a":"black"}},{"sections.questions":{"q":"size","a":"large"}}]})