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

**

in my case this $concat worked for me ...

**

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