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 } },
  { $set: { column_2: { $add: ['$column_4', '$column_3'] } } },
  false, true
)

which doesn't seem to work though, and throws not ok for storage.

I've also tried this:

db.collection.update(
  { "_id": { $exists : true } },
  { $set: { column_2: { $add: ['a', 'b'] } } },
  false, true
)

but even this shows the same error not ok for storage.

I want to concatenate only on the mongo server and not in my application.

Answer 1

Unfortunately, MongoDB currently does not allow you to reference the existing value of any field when performing an update(). There is an existing Jira ticket to add this functionality: see SERVER-1765 for details.

At present, you must do an initial query in order to determine the existing values, and do the string manipulation in the client. I wish I had a better answer for you.