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 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
}