MongoDB: concatinate multiple number values to string

Question

I have a document (inside aggregation, after $group stage) which have an object (but I could form array, if I needed it to) with number values.

MongoPlayground example with data and my aggregate query available here.

And I want to make a new _id field during next $project stage, consisted of this three number values, like:

   item_id | unix time | pointer
_id: 453435-41464556645@1829

The problem is, that when I am trying to use $concat, the query returns me an error like:

$concat only supports strings, not int

So here is my question: is it possible to achieve such results? I have seen the relevant question MongoDB concatenate strings from two fields into a third field, but it didn't cover my case.

You $concat only support string, you need to convert your fields value to string using $toString check mongoplayground.net/p/SSlXW4gIs_X — turivishal
– turivishal, Commented Aug 3, 2020 at 17:41
@turivishal that is great, thank you so much! Could you form it as an answer? So I can accept it and mark as solved? — AlexZeDim
– AlexZeDim, Commented Aug 3, 2020 at 17:50

turivishal · Accepted Answer · 2020-08-03 18:05:17Z

9

The $concat only concatenate strings, these fields $_id.item_id contains int value and $_id.last_modified double value,

The $toString converts a value to a string,

_id: {
  $concat: [
    {
      $toString: "$_id.item_id"
    },
    " - ",
    {
      $toString: "$_id.last_modified"
    }
  ]
}

answered Aug 3, 2020 at 17:58

turivishal

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