mongodb: array filtering by index or another field

Question

When filtering an array, is it possible:

filter by item index (e.g. get all array elements but the first)?
filter by value in another field within the same document that contains the array field?

Consider document:

{
   _id: 1,
   first_seen_on: 20160312,
   seen_on: [20160312, 20160313, 20160324]
}

I need the seen_on array to become [20160313, 20160324]

This doesn't seem to work (I think $first_seen_on is treated as literal value):

        'repeats': {
            '$filter': {
                'input': '$seen_on', 
                'as': 'date', 
                'cond': {'$ne': ['$date', '$first_seen_on']}
            }

when you say filtering, you refer to the filter stage in the aggregation pipeline? — lesingerouge
– lesingerouge, Commented May 11, 2016 at 6:34
Filtering an array, like in the sample code I included, in the project stage of the aggregation pipeline. — dMb
– dMb, Commented May 11, 2016 at 6:36

chridam · Accepted Answer · 2017-01-17 12:19:19Z

1

Use $setDifference as follows

db.collection.aggregate([
    {
        "$project": {
            "repeats": {
                "$setDifference": ["$seen_on", ["$first_seen_on"]]
            }
        }
    }
])

Sample Output

/* 1 */
{
    "_id" : 1,
    "repeats" : [ 
        20160313, 
        20160324
    ]
}

answered Jan 17, 2017 at 12:19

chridam

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