code: function has(id, cb) {
          // Do we have it in the local cache?
          if (sessionIdCache[id]) {
              cb(true);
          }
  
          // Looking in the database
          else {
              db.collection('trackedUsersCache').find({
                      _id: id
                  })
                  .toArray((err, docs) => {
                      if (errorHandler.dbErrorCatcher(err)) { return; }
  
                      // We did not find anything
                      if (!docs || docs.length < 1) {
                          cb(false);
                      }
  
                      // We found it. Let's cache it and return true
                      else {
                          sessionIdCache[id] = docs[0].info;
                          cb(true);
                      }
                  });
          }
      }
docs: Do we have this user's information recorded? If yes, we already tracked it at least once