MongoDB concatenate strings from two fields into a third field

Question

How do I concatenate values from two string fields and put it into a third one?

I\'ve tried this:

db.collection.update(
  { \"_id\": { $exists: true          


        

Answer 1

You could use $set like this in 4.2 which supports aggregation pipeline in update.

db.collection.update(
   {"_id" :{"$exists":true}},
   [{"$set":{"column_2":{"$concat":["$column_4","$column_3"]}}}]
)