Insert field with array size in mongo

Question

I have a documents in mongodb, containing some array. Now I need to have a field containing a quantity of items of this array. So I need to update documents adding this fiel

Answer 1

This is much easier starting with MongoDB v3.4, which introduced the $addFields aggregation pipeline operator. We'll also use the $out operator to output the result of the aggregation to the same collection (replacing the existing collection is atomic).

db.myDocuments.aggregate( [
  {
    $addFields: {
      itemsTotal: { $size: "$items" } ,
    },
  },
  {
    $out: "myDocuments"
  }
] )

WARNING: this solution requires that all documents to have the items field. If some documents don't have it, aggregate will fail with

"The argument to $size must be an array, but was of type: missing"

You might think you could add a $match to the aggregation to filter only documents containing items, but that means all documents not containing items will not be output back to the myDocuments collection, so you'll lose those permanently.