Provide proper attribute inheritance when subclassing Models

[mje113]

Currently, Couchbase::Model stores attributes (and views) in a class variable set to a mutable hash. This breaks the expected subclass attribute inheritance behavior that ActiveModel provides.

Example:

class A < Couchbase::Model
  attribute :one
end

class B < A
  attribute :two
end

B.attributes #=> {:one => nil, :two => nil}
A.attributes #=> {:one => nil, :two => nil}
B.attributes.object_id == A.attributes.object_id #=> true

This patch enables proper inheritance when ::Rails is present by using Class#class_attribute and detecting subclass additions to attribute:

class A < Couchbase::Model
  attribute :one
end

class B < A
  attribute :two
end

A.attributes #=> {:one => nil}
B.attributes #=> {:one => nil, :two => nil}
B.attributes.object_id != A.attributes.object_id #=> true

[mje113]

Ok, I managed to rewrite the patch to not rely on class_attribute. It was actually breaking some non-active model related functionality anyway.

The main change with @@attributes & @@views is instead of tracking each sub-classes' attributes in their own key on the one class variable, each subclass dupes it's parent's class or instance variable version into it's own class-level instance variable.

[mje113]

Thanks, missed some cleanup.

[avsej]

Hi Mike, sorry for delay. I've uploaded your patch into our master repository, which managed by gerrit code review tool. Could you register there and accept our CLA? After that I can merge your change. Github is just a mirror of that repository. Your patch located here:

http://review.couchbase.org/23392

Thanks for your contribution

Sergey Avseyev