code: protected final String getValue(final String key, final FieldCase ca) throws IdNotFoundException {
  		ConstraintFactory f = generator.getConstraintFactory();
  		String value = f.getModel(key).initValues(ca);
  		if (value == null) {
  			return "";
  		}
  		return value;
  	}
docs: Method searches the constraint for the given key, initializes it with the passed FieldCase
  and returns this value.
  
  @return return the value of the constrain for the passed key. If the value is {@code null}
  return an empty string