problem with delta index and custom sql attribute

[matteo folin]

I have this index definition with a custom sql attribute:

define_index do
is_active_sql = "(products.deleted_at IS NULL AND
products.available_on <= NOW())
...
has is_active_sql, :as => :is_active, :type => :boolean
....
set_property :delta => true
end

when I create a new Product delta index works fine but is_active is
not calculated and remain false.

There's a solution or I must my Product model ?

Thanks.