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Find duplicates in array field in MongoDB?

+2 votes
1,165 views

I get duplicates of fields and subfield using aggregation. Searching for duplicated sub field "k01.v" works fine:

db.table_0.aggregate({"$group" : { _id: "$k01.v", "count": { "$sum": 1 } } }
{"$match": {"count" : {"$gt": 1} } }
{"$project": {"k01.v" : "$_id", "_id" : 0} });

Unfortunately, this doesnt work when searching for sub field "a1.0" (field "a1" is an array):

db.table_0.aggregate({"$group" : { _id: "$a1.0", "count": { "$sum": 1 } } }
{"$match": {"count" : {"$gt": 1} } }
{"$project": {"a1.0" : "$_id", "_id" : 0} });

Any workaround ?

posted Apr 25, 2016 by anonymous

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2 Answers

+2 votes

try this script :

->db.table_0.find({"$arrayElemAt:["$a1",0]});
answer Apr 26, 2016 by Shivam Kumar Pandey
0 votes

Are you trying to access the first element of array "a1"? The syntax
for that in aggregation is

{$arrayElemAt:["$a1", 0] }

answer Apr 26, 2016 by Manikandan J
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