Given the index
{
"v" : 1,
"key" :

{ "pricings.parent_id" : 1, "start_at" : 1 }

,
"ns" : "accounts.calls",
"name" : "pricings.parent_id_1_start_at_1"
}

The query
{
"start_at":

{ "$gte": ISODate("2012-10-15"), "$lt": ISODate("2012-10-15 08:00") }

,
"pricings.parent_id": 4
}
fails to use the upper bounds for start_at.

The explain() is;
{
"cursor" : "BtreeCursor pricings.parent_id_1_start_at_1",
"isMultiKey" : true,
"n" : 2526,
"nscannedObjects" : 349468,
"nscanned" : 349468,
"nscannedObjectsAllPlans" : 349468,
"nscannedAllPlans" : 349468,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 550,
"indexBounds" :

{ "pricings.parent_id" : [ [4,4] ], "start_at" : [ [ ISODate("2012-10-15T00:00:00Z"), ISODate("292278995-01--2147483647T07:12:56.808Z") ] ] }

,
"server" : "onatopp:27017"
}