Match Whole Array in Aggregation

Question

I have a simple mongodb collection (an extract of aggregation pipeline), say

[
 {_id : [3, 3] ,  val : 7},
 {_id : [3, 2] ,  val : 9},
 {_id : [1, 3] ,  val: 10}
]

I want to do a $match to get the [3, 3] record only. However using

$match : {$_id: { $all: [3, 3]}}

returns [3, 2] as well, which is what we don't want. Any takers?

i think you missed the part of my question saying that its an extract of aggregation pipe line, i can be an array thus — fahad daniyal
– fahad daniyal, Commented Sep 29, 2015 at 14:23
I think you missed the part of your question to show your workings and proof. — Blakes Seven
– Blakes Seven, Commented Sep 29, 2015 at 14:23

Blakes Seven · Accepted Answer · 2015-09-29 14:35:46Z

I can only see this possibly occurring under a condition like this:

{ "like": 1, "id": 3, "val": 7 },
{ "like": 1, "id": 3, "val": 7 },
{ "like": 2, "id": 3, "val": 5 },
{ "like": 2, "id": 2, "val": 5 },

And then in aggregate:

db.collection.aggregate([
    { "$group": {
        "_id": "$like",
        "id": { "$push": "$id" },
        "val": { "$max": "$val" }
    }},
    { "$project": {
        "_id": "$id",
        "val": 1
    }}
])

Which gives:

{ "_id" : [ 3, 2 ], "val" : 5 }
{ "_id" : [ 3, 3 ], "val" : 7 }

For which we come down to, the $all condition is basically considered as a "set" just as $in is. And that means [3,3] is basically reduced to 3. Therefore if you want an "explicit" match then please do so:

db.collection.aggregate([
    { "$group": {
        "_id": "$like",
        "id": { "$push": "$id" },
        "val": { "$max": "$val" }
    }},
    { "$project": {
        "_id": "$id",
        "val": 1
    }},
    { "$match": {
        "_id": [3,3]
    }}
])

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