'Returns true if the projection query should be distinct.
This is equivalent to the SQL syntax: SELECT DISTINCT. It is only available
for projection queries, it is not valid to specify distinct without also
specifying projection properties.
Distinct projection queries on entities with multi-valued properties will
return the same entity multiple times, once for each unique combination of
properties included in the projection.
Returns:
True if this projection query is distinct.'

def is_distinct(self):

    raise NotImplementedError
