Finding duplicate values in a MongoDB array

Question

I have a collection containing entries in following format:

{ 
    "_id" : ObjectId("5538e75c3cea103b25ff94a3"), 
    "userID" : "USER001", 
    "userName" : "manish", 
    "collegeIDs" : [
        "COL_HARY",
        "COL_MARY",
        "COL_JOHNS",
        "COL_CAS",
        "COL_JAMES",
        "COL_MARY",
        "COL_MARY",
        "COL_JOHNS"
    ]
}

I need to find out the collegeIDs those are repeating. So the result should give "COL_MARY","COL_JOHNS" and if possible the repeating count. Please do give a mongo query to find it.

Answer 1

Probably there would be many of these documents and thus you want it per ObjectId.

db.myCollection.aggregate([
  {"$project": {"collegeIDs":1}},
  {"$unwind":"$collegeIDs"},
  {"$group": {"_id":{"_id":"$_id", "cid":"$collegeIDs"}, "count":{"$sum":1}}},
  {"$match": {"count":{"$gt":1}}},
  {"$group": {"_id": "$_id._id", "collegeIDs":{"$addToSet":"$_id.cid"}}}
])

This might be what you want to, not clear from your question:

db.myCollection.aggregate([
  {"$match": {"userID":"USER001"}},
  {"$project": {"collegeIDs":1, "_id":0}},
  {"$unwind":"$collegeIDs"},
  {"$group": {"_id":"$collegeIDs", "count":{"$sum":1}}},
  {"$match": {"count":{"$gt":1}}},
])