How to memoize a class method in Rails?

Question

I have a model with a class method which is heavy in computation that I call many times per request.

Ideally i'd like to cache the result for the duration of a single request.

What's rails best practice in this situation?

Example:

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class MyClass < ActiveRecord::Base
  def self.heavy_method; ... ; end
end

Then in a helper

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def helper
  MyClass.heavy_method
end

This helper is used in many views

Answer 1

Here is a very generic solution that might work for you.

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class Klass
  def self.memoized_expensive_method
    @memoized_expensive_method_result ||= expensive_method
  end

  def self.expensive_method
    # ...
  end
end

Then if you want to ensure your code is re-executed on every request, you could use a filter in your controller.

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class Klass
  def self.reset_expensive_method_cache!
    @memoized_expensive_method_result = nil
  end
end

class ApplicationController
  before_filter :reset_klass_expensive_method_cache

  def reset_klass_expensive_method_cache
    Klass.reset_expensive_method_cache!
  end
end

Note that storing stuff in class variable might lead to thread-safety issue because the cached result will be shared between threads.

If this can be an issue for your application you might want to store the data in the thread instead of using a class variable.