Sunspot_rails order by not working

Hi Guys,
I’m using sunspot_rails-2.0.0.pre.120417 with postgres
In model i added searchable columns:
searchable do
string :display_title
integer :created_at
integer :ending_at
end

order_by(:created_at,:desc) not working.

ruby-1.9.2-p180 :027 > res=Listing.search{order_by(:created_at,:desc)}
=> Sunspot::Search:{:fq=[“type:Listing”], :sort=>“created_at_d desc”, :start=>0, :rows=>30, :q=>":"}>

ruby-1.9.2-p180 :028 > res.results.map(&:id)
Listing Load (3.0ms) SELECT “listings”.* FROM “listings” WHERE “listings”.“id” IN (100, 99, 94, 95, 96, 97, 98, 93, 88, 89, 90, 91, 92, 83, 84, 85, 86, 87, 77, 78, 79, 80, 81, 82, 72, 73, 74, 75, 76, 67)
=> [100, 99, 94, 95, 96, 97, 98, 93, 88, 89, 90, 91, 92, 83, 84, 85, 86, 87, 77, 78, 79, 80, 81, 82, 72, 73, 74, 75, 76, 67]

ruby-1.9.2-p180 :029 > ActiveRecord::Base.connection.execute(‘select listings.id FROM “listings” order by created_at desc limit 30’).column_values(0)
(0.9ms) select listings.id FROM “listings” order by created_at desc limit 30
=> [“100”, “99”, “98”, “97”, “96”, “95”, “94”, “93”, “92”, “91”, “90”, “89”, “88”, “87”, “86”, “85”, “84”, “83”, “82”, “81”, “80”, “79”, “78”, “77”, “76”, “75”, “74”, “73”, “72”, “71”]

Its working during sql order by and order_by(:ending_at,:desc) but not for order_by(:created_at,:desc), i dont know why, Any suggestion?

Thanks,
Dhamodharan.N