Scoped aggregation?

[mattiassvedhem]

I've wanted to do something like this

class Post
  include Mongoid::Document
  include Mongoid::TaggableWithContext
  include Mongoid::TaggableWithContext::AggregationStrategy::RealTime
  field :state
  taggable
end

Then to be able to do:

Post.create(state: 'published', tags: 'coffeescript')
Post.create(state: 'published', tags: 'ruby')
Post.create(state: 'unpublished', tags: 'java')

Post.where(state: 'published').tags
=> ['ruby', 'coffeescript']

Any ideas on how to implement?