How to convert an array of array to an array of single value with updateMany()?

Working with Data
, ,
2

You were on the right track with $reduce. The enclosing $expr and $gt were not. The $add was not the correct operator. Try:

{ "$set" : {
  "barcodes" :
  {
    "$reduce" :
    {
       "input" : "$barcodes" ,
       "initialValue" : [] ,
       "in" : { "$concatArrays" : [
           "$$value' ,
           { "$cond" : [
             { "$eq" : [ {"$type" : "$$this"} , "array" ] } ,
             "$$this" ,
             [ "$$this" ]
           ] }
       ] }
    }
  }
} }

In migration scenarios like yours, what I like to do is to migrate $out into a temporary collection, which I can verify before $merge-ing it back to the real collection. But you need downtime because between the $out and $merge, original data may change.