martinciu's dev blog

Deprecation warning

After you upgrade to Rails 3.1 you may be attacked by deprecation warnings similar to this one:

DEPRECATION WARNING: class_inheritable_attribute is deprecated, please use class_attribute method instead. Notice their behavior are slightly different, so refer to class_attribute documentation first. (called from included at base.rb:2)

In most case it will probably be all right to replace your all class_inheritable_hash, class_inheritable_array, etc method with class_attribute. Unfortunately it won’t always work. There is a “slight” difference mentioned by warning above.

What is the difference?

With class_inheritable_hash (the old way):

class Base
  class_inheritable_hash :mappings
  self.mappings = {}
end

class FirstChild < Base
  self.mappings[:foo] = :bar
end

class SecondChild < Base
  self.mappings[:bar] = :baz
end

Base.mappings           # {}
FirstChild.mappings     # {:foo=>:bar}
SecondChild.mappings    # {:bar=>:baz}

Alright, this is the expected behavior. How does this code works with class_attribute? Let’s see!

class Base
  class_attribute :mappings
  self.mappings = {}
end

class FirstChild < Base
  self.mappings[:foo] = :bar
end

class SecondChild < Base
  self.mappings[:bar] = :baz
end

Base.mappings           # {:foo=>:bar, :bar=>:baz}
FirstChild.mappings     # {:foo=>:bar, :bar=>:baz}
SecondChild.mappings    # {:foo=>:bar, :bar=>:baz}

It is far from the how it supposed to work. I would’t say that class_attribute behavior is “slightly” different than “class_inheritable_hash”. It is completely different method! But there is no place for complaining. We have to deal with it. How can we do it? Actually quite simple. Just call dup method on an inheritable attribute:

class Base
  class_attribute :mappings
  self.mappings = {}
end

class FirstChild < Base
  self.mappings = self.mappings.dup
  self.mappings[:foo] = :bar
end

class SecondChild < Base
  self.mappings = self.mappings.dup
  self.mappings[:bar] = :baz
end

Base.mappings           # {}
FirstChild.mappings     # {:foo=>:bar}
SecondChild.mappings    # {:bar=>:baz}

Exactly as expected! Hooray!