'Given a lookup_type of \'year\', \'month\', or \'day\', return the SQL that
truncates the given date field field_name to a date object with only
the given specificity.'

def date_trunc_sql(self, lookup_type, field_name):

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