Rails 3 Order By Count on has_many :through

I have an application where I can list Items and add tags to each Item. The models Items and Tags are associated like this:

class Item < ActiveRecord::Base
  has_many :taggings
  has_many :tags, :through => :taggings
end

class Tagging < ActiveRecord::Base
  belongs_to :item
  belongs_to :tag
end

class Tag < ActiveRecord::Base
  has_many :taggings
  has_many :items, :through => :taggings
end

So, this many-to-many relationship allows me to set n tags for each Item, and the same tag can be used several times.

I'd like to list all tags ordered by the number of items associated with this tag. More used tags, shows first. Less used, last.

How can I do that?

Regards.