How can I cache a calculated column in rails?

Question

I have a tree of active record objects, something like:

class Part < ActiveRecord::Base
  has_many :sub_parts, :class_name => \"Part\"

  def complicat         


        

Answer 1

1. You can stuff the actually cached values in the Rails cache (use memcached if you require that it be distributed).

2. The tough bit is cache expiry, but cache expiry is uncommon, right? In that case, we can just loop over each of the parent objects in turn and zap its cache, too. I added some ActiveRecord magic to your class to make getting the parent objects simplicity itself -- and you don't even need to touch your database. Remember to call Part.sweep_complicated_cache(some_part) as appropriate in your code -- you can put this in callbacks, etc, but I can't add it for you because I don't understand when complicated_calculation is changing.

   class Part < ActiveRecord::Base
     has_many :sub_parts, :class_name => "Part"
     belongs_to :parent_part, :class_name => "Part", :foreign_key => :part_id
   
     @@MAX_PART_NESTING = 25 #pick any sanity-saving value
   
     def complicated_calculation (...)
       if cache.contains? [id, :complicated_calculation]
         cache[ [id, :complicated_calculation] ]
       else
         cache[ [id, :complicated_calculation] ] = complicated_calculation_helper (...)
       end
     end
   
     def complicated_calculation_helper
       #your implementation goes here
     end
   
     def Part.sweep_complicated_cache(start_part)
       level = 1  # keep track to prevent infinite loop in event there is a cycle in parts
       current_part = self
   
       cache[ [current_part.id, :complicated_calculation] ].delete
       while ( (level <= 1 < @@MAX_PART_NESTING) && (current_part.parent_part)) {
        current_part = current_part.parent_part)
        cache[ [current_part.id, :complicated_calculation] ].delete
       end
     end
   end