I have 2 models User and Discussion. I have defined the indices for the models as below: For the User model: define_index do indexes email indexes first_name indexes last_name, :sortable => true indexes groups(:name), :as => :group_names has "IF(email_confirmed = true and status = 'approved', true, false)", :as => :approved_user, :type => :boolean has "IF(email_confirmed = true and (status = 'approved' or status='blocked'), true, false)", :as => :approved_or_blocked_user, :type => :boolean has points, :type => :integer has created_at, :type => :datetime has user(:id) set_property :delta => true end For the Discussion model: define_index do indexes title indexes description indexes category(:title), :as => :category_title indexes tags(:title), :as => :tag_title has "IF(publish_to_blog = true AND sticky = false, true, false)", :as => :publish_to_main, :type => :boolean has created_at has updated_at, :type => :datetime has recent_activity_at, :type => :datetime has views_count, :type => :integer has featured has publish_to_blog has sticky set_property :delta => true end I have added a delta column to both tables as per the documentation. My problem is that delta indexing works only for the Discussion model and not for the User model. For ex: When I update the 'title' of a discussion, I can see the thinking sphinx is rotating the indices etc. (as is evident from the logs). But when I update the 'first_name' or the 'last_name' of a user, nothing happens. The User model also has a has_many :through association through a model called GroupsUser. I have setup a after_save on the GroupsUser as follows: def set_user_delta_flag user.delta = true user.save end Even this doesn't seem to trigger delta indexing on the User model. A similar setup for the Discussion model works perfectly! Can anyone tell me why this is happening?