db={"stocks":[{_id:1,"userId":1,"groupId":1,"stockId":1},{_id:2,"userId":2,"groupId":1,"stockId":2},{_id:3,"userId":3,"groupId":4,"stockId":3}],items:[{_id:4,"userid":1,"groupId":1,"itemId":1},{_id:5,"userid":1,"groupId":3,"itemId":2},{_id:6,"userid":1,"groupId":4,"itemId":3}]}

db.stocks.aggregate([{$match:{$expr:{$ne:["$groupId","$userId"]}}},{"$unionWith":{"coll":"items","pipeline":[{$match:{$expr:{$ne:["$groupId","$userId"]}}}]}}])