code: function validateUniqueness(attr, conf, err, done) {
      var cond = {where: {}};
      cond.where[attr] = this[attr];
      this.constructor.all(cond, function (error, found) {
          if (found.length > 1) {
              err();
          } else if (found.length === 1 && (!this.id || !found[0].id || found[0].id.toString() !== this.id.toString())) {
              err();
          }
          done();
      }.bind(this));
  }
docs: Uniqueness validator
  @param {mixed} attr
  @param {mixed} conf
  @param {mixed} err
  @param {Function} done