If you use a leaf from grouping of type union as a key in list, binding generates key class without that leaf as part of key.

grouping common-cars {
  leaf brand {
    type union {
      type uint64;
      type string;
    }

  }
  leaf engine { type string; }
  leaf gearbox { type string; }
}
container cars {
  list car {
    key "brand";
    uses common-cars;
  }

}

From this yang we get empty key binding class

public class CarKey
  implements Identifier<Car> {
  private static final long serialVersionUID = -7229666078144872030L;
  public CarKey() {
  }
}

Simple workaround is to not use grouping. You can also define your own type with the appropriate union type as the base type and use this new type as a type for leaf in grouping.