Rails: auto increment column with scope

Question

I have projects and issues. The issue number (not ID) must be auto-increment but scoped with project_id. Any gem or easy way to accomplish that? Thank you!

Edit: acts_as_list will do the job, but what if a record is deleted? The next issue will be with it's number.

Answer 1

in your Issue class:

belongs_to :project
before_validation( :on => :create ) do 
  self.issue_number = self.project.issues.collect { | issue | issue.issue_number }.max + 1
end

(or thereabouts) -- essentially, before the new object is created, find the max issue number of the issues associated with the issue's project, increment that, and use it for the new issue number...