Named scopes in Rails are just awesome. They so much DRY and beautify your code! But did you ever had the trouble to create a new named_scope out of others? E.g. aliasing multiple ones to a new one. Luckily I now figured out how this works in a nice way.
The magic key is the method scope( :find )

This method called on a Scope class returns the Hash used for the ActiveRecord .find() call. First it’s a nice way to debug your scopes, second it’s perfect for our need to chain multiple scoped into one. Check this out:

[ruby]
class Fu < ActiveRecord::Base
named_scope :has_moo, :conditions => { :moo => true }
named_scope :has_foo, :conditions => { :foo => true }

named_scope :has_moo_and_foo, lambda { has_moo.has_foo.scope(:find) }
end
[/ruby]

We chain the two scoped has_moo and has_foo together into a new one called has_moo_and_foo. As the return value can only be a Hash we use the above mentioned scope(:find) to transform it into one.

Now this works:

[ruby]Fu.has_moo_and_foo.all[/ruby]

Sweet! Imagine how great this would be if we can return a scope object instead of a hash as well??

See my pastie here too.