sorting - Mongodb sort on conditional clause -

i want this:-

db.pub_pairs.find({status:'active'})             .sort( { if (pcat="abc") 0 else 1 end })             .limit(10) 

in other words, want prefer records field pcat "abc". need fast, can add indexes necessary. suggestions on how this? thanks

a decently efficient way of doing query db twice. once documents having pcat="abc" , once (if needed) documents having pcat!="abc"

something along lines of (pseudo-code):

var data = db.pub_pairs.find({status:'active', pcat="abc"})                        .limit(10) if (data.length < 10)   data.concat(db.pub_pairs.find({status:'active', pcat!="abc"})                           .limit(10-data.length)) 

for perform well, suggest compound index on {status: 1, pcat: 1}

please note, like other "find" mongodb, have prepared retrieve same document twice in result set, notably here if pcat field concurrently modified other client during execution.