[{"limit":{"custom:key:a":{"role":"admin","user_id":"user:1"}}},{"limit":{"custom:key:b":{"role":"admin","user_id":"user:2"}}},{"limit":{"custom:key:c":{"role":"user","user_id":"user:3"}}}]

db.collection.find({$or:[{_id:ObjectId("5a934e000102030405000000")},{_id:ObjectId("5a934e000102030405000002")}]})