blimmer · September 27, 2017 15:04 · Sep 27, 2017
diff --git a/foo_for_caching.rb b/foo_for_caching.rb
@@ -0,0 +1,80 @@
+# 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