Question

I have a class in which looking up an instance is expensive, so instances are cached:

class Foo   def self.find(id)     Rails.cache.fetch('Foo.#{id}') do       // some expensive lookup, like an HTTParty request, or a long SQL query       ...     end   end end 

That works fine until Foos have related Foos:

class Foo   def children     @child_foo_ids.map { |id| Foo.find(id) }   end end 

I’d like to use ||= caching to save repeated trips:

class Foo   def children     @children ||= @child_foo_ids.map { |id| Foo.find(id) }   end end 

But Rails.cache freezes the found Foos, so I can’t set the instance variable after I create and cache the object. (i.e. this method raises a TypeError.)

One solution would be to pre-fetch the parent when I first do the expensive find, but that could end up loading a gigantic object graph when I only want one or two instances.

Answer 1

You can use ||= caching; you just have to use a little indirection:

class Foo   def initialize     @related_objects = {}   end   def children     @related_objects[:children] ||= @child_foo_ids.map { |id| Foo.find(id) }   end end 

Rails.cache won’t freeze the instance variables of each Foo, so that Hash can be modified!

PS: yes, I did just post this question and the answer at essentially the same time. I figured the community could benefit from my struggle.