title: Elasticsearch + Logstash: How to add a fields based on existing data at importing time
body: <p>Currently, I'm importing data into Elastic through logstash, at this time by reading csv files.</p>
  
  <p>Now let's say I have two numeric fields in the csv, age, and weight.
  I would need to add a 3rd field on the fly, by making a math on the age, the weight and another external data ( or function result ); and I need that 3rd field to be created when importing the data.</p>
  
  <p>There is any way to do this? 
  What could be the best practice?</p>