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

Building on the answer from @rebe100x, as suggested by @Jamby ...

You can use $project, $concat and $out (or $merge) in an aggregation pipeline. https://docs.mongodb.org/v3.0/reference/operator/aggregation/project/ https://docs.mongodb.org/manual/reference/operator/aggregation/concat/ https://docs.mongodb.com/manual/reference/operator/aggregation/out/

For example:

    db.collection.aggregate(
       [
          { $project: { newfield: { $concat: [ "$field1", " - ", "$field2" ] } } },
          { $out: "collection" }
       ]
    )

With MongoDB 4.2 . . .

MongoDB 4.2 adds the $merge pipeline stage which offers selective replacement of documents within the collection, while $out would replace the entire collection. You also have the option of merging instead of replacing the target document.

    db.collection.aggregate(
       [
          { $project: { newfield: { $concat: [ "$field1", " - ", "$field2" ] } } },
          { $merge: { into: "collection", on: "_id", whenMatched: "merge", whenNotMatched: "discard" }
       ]
    )

You should consider the trade-offs between performance, concurrency and consistency, when choosing between $merge and $out, since $out will atomically perform the collection replacement via a temporary collection and renaming.

https://docs.mongodb.com/manual/reference/operator/aggregation/merge/ https://docs.mongodb.com/manual/reference/operator/aggregation/merge/#merge-out-comparison

Answer 2

**

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

**

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