'Given a lookup_type of \'hour\', \'minute\' or \'second\', return the SQL
that truncates the given time field field_name to a time object with
only the given specificity.'

def time_trunc_sql(self, lookup_type, field_name):

    raise NotImplementedError('subclasses    of    BaseDatabaseOperations    may    require    a    time_trunc_sql()    method')
