Database
[{"_id":ObjectId("52fe4456c3a368484e01d6bd"),"c_name":"test_name","m_id":ObjectId("4bc8c421017a3c122d083836"),"p_id":ObjectId("4bc88b21017a3c122d001059"),"c_ids":[ObjectId("61c171a7907f26002d6485c8"),ObjectId("61c171a7907f26002d6485c9")]}]
Query
db.collection.update({"_id":ObjectId("52fe4456c3a368484e01d6bd")},{$addToSet:{"c_ids":ObjectId("61c171a7907f26002d6485c9")}})