code: function applyTTL (cond) {
      if (cond[key]) {
        cond.$and || (cond.$and = []);
        var a = {};
        a[key] = cond[key];
        cond.$and.push(a);
        var b = {};
        b[key] = { $gt: new Date };
        cond.$and.push(b);
        delete cond[key];
      } else {
        cond[key] = { $gt: new Date };
      }
    }
docs: Applies ttl to query conditions.
  @private