How to copy field data and use it in another field of same document using mongoose?

I have two fields in my User Scehma , cart and orders both are array.

I want to copy all the items in cart array to orders array and then remove/delete all the items in my cart array. How do I that ?

>Solution :

use update $set with $concatArrays

db.collection.update({},
[
  {
    $set: {
      orders: {
        $concatArrays: [
          "$orders",
          "$carts"
        ]
      },
      carts: []
    }
  }
])

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