acts_as_shareable.rb

def shared_to?(object, by_user=nil)
  shareable = ActiveRecord::Base.send(:class_name_of_active_record_descendant, self.class).to_s
  to = ActiveRecord::Base.send(:class_name_of_active_record_descendant, object.class).to_s
  if by_user
    s = Share.find(:first, :conditions=>["shareable_type = ? and shareable_id = ? and shared_to_type = ? and shared_to_id = ? and user_id=?",
                  shareable, id, to, object.id, by_user.id])
  else
    s = Share.find(:first, :conditions=>["shareable_type = ? and shareable_id = ? and shared_to_type = ? and shared_to_id = ?",
                  shareable, id, to, object.id])
  end
  return !s.nil?
end