Rails Issue 30680 Workaround

foo_for_caching.rb

# This class exists to make the caching of Foo objects more efficient.
# Marshaling full activerecord objects is expensive, especially in Rails 5
# because of this bug: https://github.com/rails/rails/issues/30680
# This simple ruby class selects a very small subset of parameters needed
# for common operations on these objects. It also acts like Foo, by
# passing attributes through to an in-memory Foo model.
class FooForCaching
  attr_accessor :attributes

  # This is a list of attributes we want to save in cache
  ATTRIBUTES_CACHED = %i(
    id
    bar
    baz
  ).freeze

  # These are methods that we've ensured we have the cached attributes to properly
  # calculate.
  SAFE_PASSTHROUGH_METHODS = %i(
    my_method_1
    my_method_2
  ).concat(ATTRIBUTES_CACHED).freeze

  # Constructor
  # @param [Hash] attributes the attributes to cache. attributes not provided will be interpreted to be `nil`
  def initialize(attributes)
    @attributes = attributes.symbolize_keys.slice(*ATTRIBUTES_CACHED)
  end

  # Don't create the in-memory activerecord model to access the simple id
  # attribute.
  def id
    attributes[:id]
  end

  # Pass undefined attribute methods to a memoized, in-memory Foo object
  # to allow running methods on an ActiveRecord Foo object.
  def method_missing(method, *args)
    unless SAFE_PASSTHROUGH_METHODS.include?(method)
      log_unsafelisted_method_passthrough(method)
    end

    if respond_to_missing?(method)
      ar_model_with_attrs.public_send(method, *args)
    else
      super
    end
  end

  # See https://rubocop.readthedocs.io/en/latest/cops_style/#stylemethodmissing
  def respond_to_missing?(method, *args)
    ar_model_with_attrs.public_methods.include?(method) || super(method, args)
  end

  # Do not accidentally cache the in-memory ActiveRecord object if it was created
  # before the Marshal dump was called. Caching that object would re-introduce
  # the problem that this class intends to solve.
  def marshal_dump
    @attributes
  end

  # Set the attributes instance variable when this object is unmarshalled from
  # cache.
  def marshal_load(attributes)
    @attributes = attributes
  end

  private

  def ar_model_with_attrs
    @ar_model = Foo.new(@attributes).tap(&:readonly!)
  end

  # exposed for testing
  def log_unsafelisted_method_passthrough(method)
    Rails.logger.warn(
      "[FooForCaching] Method #{method} called. This method will be delegated, but it's not explicitly marked as safe."
    )
  end
end