Redmine

I have created a custom field. The problem I have is that this field is based off of other fields.

Example is custom_field = tracker_id + project_id + id

This needs to be created when a new issue is created. New to Ruby and Ruby on Rails. Just not sure how to do this, where to put this code, is it a plugin, do I use a hook ect.

Sys. Info
- Redmine 2.2.2
- Apache 2.2.23
- ImageMagick 6.5.6-8-Q8
- MySQL 5.5.29
- Subversion 1.7.5
- Git 1.7.11
- Ruby 1.9.3-p194-i386-mingw32
- Rails 3.2.11
- RubyGems 1.8.12