'Return true if this element references a member which meets the
given criterion.
The usual implementation of ``has()`` is
:meth:`.RelationshipProperty.Comparator.has`.
:param criterion: an optional ClauseElement formulated against the
member class\' table or attributes.
:param \**kwargs: key/value pairs corresponding to member class
attribute names which will be compared via equality to the
corresponding values.'

def has(self, criterion=None, **kwargs):

    return self.operate(PropComparator.has_op, criterion, **kwargs)
