code: public void validateParentKey(Key parentKey) {
  		if (parents.isEmpty()) {
  			if (parentKey != null) {
  				throw new IllegalArgumentException("Specified parent key " + parentKey + ", but entity " + this.persistentClass.getSimpleName() + " is configured as a root class (missing @Id(parent)?)");
  			}
  		} else {
  			if (parentKey == null) {
  				throw new IllegalArgumentException("Missing parent key for entity " + this.persistentClass.getSimpleName() + ". Expected: " + Joiner.on(", ").join(parents));
  			}
  			if (!parents.contains(parentKey.getKind())) {
  				throw new IllegalArgumentException("Specified parent key " + parentKey + ", but entity " + this.persistentClass.getSimpleName() + " expects parents with type " + Joiner.on(", ").join(parents));
  			}
  		}
  	}
docs: Validates the parent key when inserting