Adding an autoincrementing SNO column in Rails Scaffolding?

Question

Is there a way in which I can create a serial number (SNO) column through scaffolding in Rails which increases on adding a record, decreases on deleting a record and cannot be modified manually?

Answer 1

If you want to create a summary column on a model you will need to put this logic into your models. There is not a built in method for this (like a standard autoincrement field), but it can be added easily:

class Parent << ActiveRecord::Base 
    # Contains a field: summary_field
end 


class Child << ActiveRecord::Base

   after_save => :increment_summary
   before_destroy => :decrement_summary

   def increment_summary
        Parent.find(self.id).summary_field.increment
   end 

   def decrement_summary
        Parent.find(self.id).summary_field.decrement
   end 

end

Answer 2

It's not clear whether there is any relationship involved, but it sounds like counter_cache may be a good fit.

A Railscast episode provides the code examples and a video tutorial.