Convert ObjectID to String in mongo Aggregation

Question

I\'m in this scenario right now: I have a collection X:

{
  _id:ObjectId(\'56edbb4d5f084a51131dd4c6\'),
  userRef:ObjectId(\'56edbb4d5f084a51131dd4c6\'),
  seria         


        

Answer 1

You can simply use $toString to apply $concat in aggregation on ObjectIDs in the following way -

$group: {
    '_id': {
        '$concat': [
            { '$toString' : '$userRef' },
            '-',
            { '$toString' : '$serialNumber'}
        ]
    },
}

Answer 2

I think you may try to resolve it by using an Array which contains both fields:

{$project:{newkey:['$userRef','$serialNumber']},{$match:{newkey:{$in:filterArray}}}}

this may match the data with both fields to the filter. Please notice that the data in the newkey array should have the same data type with the filterArray elements.

Answer 3

Now you can try with $toString aggregation which simply converts ObjectId to string

db.collection.aggregate([
    { "$addFields": {
        "userRef": { "$toString": "$userRef" }
    }},
    { "$group": {
      "_id": { "$concat": ["$userRef", "-", "$serialNumber"] }
    }}
])

You can check the output here

Answer 4

I couldn't find a way to do what I wanted, so instead, I created a MapReduce function that, in the end, generated the keys the way I wanted to (concatenating other keys).

At the end, it looked something like this:

db.collection('myCollection').mapReduce(
    function() {
        emit(
            this.userRef.str + '-' + this.serialNumber , {
                count: 1,
                whateverValue1:this.value1,
                whateverValue2:this.value2,
                ...
            }
        )
    },
    function(key, values) {
       var reduce = {}
       .... my reduce function....
        return reduce
    }, {
        query: {
            ...filters_here....
        },
        out: 'name_of_output_collection'
    }
);

Answer 5

You can use $substr https://docs.mongodb.com/manual/reference/operator/aggregation/substr/#exp._S_substr to cast any object to string before $concat.

This is a sample of code that's working for me.

group_id_i['_id'] = {
    '$concat' => [
        { '$substr' => [ {'$year' => '$t'}, 0, -1] }, '-',
        { '$substr' => [ {'$month' => '$t'}, 0, -1] }, '-',
        { '$substr' => [ {'$dayOfMonth' => '$t'}, 0, -1] }
    ]
} 

Where t is DateTime field, this aggregation returns data like so.

{
    "_id" => "28-9-2016",
    "i" => 2
}