NiFI 1.3+ : How to use Record Processor to Create a new Field

[#1]

I want to create a new field in the output that is the concatenation of two input fields, example

ouput schema

name

input schema

firstname

lastname

name = firstname (space) lastname

How could I do that with just the record processors?

[#2 · Accepted solution]

https://github.com/apache/nifi/pull/1920/files

Didn't see docs for it but

concat(/last_name,', ',/first_name) works