docs: function (sid, callback) {
          var cb = callback || _.noop;
  
          // ensureIndex is already called on start-up
          mongooseModel.findOne(_applyProp(propSid, sid), function(err, doc) {
  
            if(err || !doc) { // exit immediately
              cb(err, null);
              return;
            }
  
            if(_checkExpiration(doc) && doc[propData] !== undefined) { // we're still valid:
              cb(null, unserializer(doc[propData]));
            }
            else { // needs clean-up
              me.destroy(sid, cb);
            }
          });
        }