How to filter array in subdocument with MongoDB

Question

asked Feb 14, 2020 in Web Technology by ashely (50.2k points)

The following is my subdocument:

{
    "_id" : ObjectId("512e28984815cbfcb21646a7"),
    "list" : [
        {
            "a" : 1
        },
        {
            "a" : 2
        },
        {
            "a" : 3
        },
        {
            "a" : 4
        },
        {
            "a" : 5
        }
    ]
}

I want to filter the above subdocument as a>3. I want the following result:

{
    "_id" : ObjectId("512e28984815cbfcb21646a7"),
    "list" : [
        {
            "a" : 4
        },
        {
            "a" : 5
        }
    ]
}

I have tried the following implementation but it only returns one element:

db.test.find( { _id" : ObjectId("512e28984815cbfcb21646a7") }, {
    list: {
        $elemMatch:
            { a: { $gt:3 }
            }
    }
} )

The output is as follows:

{ "_id" : ObjectId("512e28984815cbfcb21646a7"), "list" : [ { "a" : 4 } ] }

Can someone guide me?

1 Answer

vinita · Answer 1 · 2020-02-14T09:39:56+0000

The approach I would suggest is aggregation. First of all, you have to perform $unwind on the list array and then implement $match. It will filter each and every element of the array and then after filtering, put it back together with the help of $group. For implementing the above-suggested tasks, refer to the following code:

db.test.aggregate([
    { $match: {_id: ObjectId("512e28984815cbfcb21646a7")}},
    { $unwind: '$list'},
    { $match: {'list.a': {$gt: 3}}},
    { $group: {_id: '$_id', list: {$push: '$list.a'}}}
])

The output is aas follows:

{
  "result": [
    {
      "_id": ObjectId("512e28984815cbfcb21646a7"),
      "list": [
        4,
        5
      ]
    }
  ],
  "ok": 1
}

If you are a beginner and want to know more about MongoDB, then check out the mongodb tutorial.

How to filter array in subdocument with MongoDB

1 Answer

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