1.9.3p125 :061 > Keyword.where(:t => 3.3).avg(:t)
=> 115.46699999999579
1.9.3p125 :062 > Keyword.where(:t.gt => 10).count
=> 0
1.9.3p125 :063 > Keyword.where(:t.lt => 0).count
=> 0
1.9.3p125 :064 > Keyword.where(:t => 3.3).count
=> 3499
1.9.3p125 :065 > Keyword.where(:t => 3.3).avg(:t)
=> 115.46699999999579
1.9.3p125 :092 > Keyword.where(:t => 7.6).avg(:t)
=> 7.599999999999999
1.9.3p125 :093 > Keyword.where(:t => nil).count
=> 0
1.9.3p125 :066 > Keyword.where(:t => 3.3).explain
=> {"cursor"=>"BasicCursor", "isMultiKey"=>false, "n"=>3499, "nscannedObjects"=>143773, "nscanned"=>143773, "nscannedObjectsAllPlans"=>143773, "nscannedAllPlans"=>143773, "scanAndOrder"=>false, "indexOnly"=>false, "nYields"=>0, "nChunkSkips"=>0, "millis"=>173, "indexBounds"=>{}, "allPlans"=>[{"cursor"=>"BasicCursor", "n"=>3499, "nscannedObjects"=>143773, "nscanned"=>143773, "indexBounds"=>{}}], "server"=>"..."}

I'm using latest version of mongoid (3.0 Oct 15th, 2012) and mongodb (2.2 August 29th 2012)