Use $push to add an existing document field to an array to the same document

Question

I have a document with fields "MyElement" and "MyArray". I want to update my entire collection so that MyElement also appears as the initial item of MyArray.

I tried the following, but it did not work: .updateMany({}, {$push:{MyArray:{$each:["$MyElement"],$position: 0}}})

This just pushed the string literal $MyElement, not the actual value. How do I refer to the actual value?

jQueeny · Accepted Answer · 2024-08-11 22:31:20Z

2

You could just use $concatArrays inside of an Updates with Aggregation Pipeline. That will allow the $MyElement variable to be parsed as the actual value and not the string literal. The aggregation pipeline is what allows you to use the variable.

It might look like:

db.collection.updateMany({},
[
  {
    $set: {
      "MyArray": {
        $concatArrays: [
          [
            "$MyElement"
          ],
          "$MyArray"
        ]
      }
    }
  }
]);

See HERE for a working example.

answered Aug 11, 2024 at 22:31

jQueeny

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