operations.py 文件源码

def check_expression_support(self, expression):
        bad_fields = (fields.DateField, fields.DateTimeField, fields.TimeField)
        bad_aggregates = (aggregates.Sum, aggregates.Avg, aggregates.Variance, aggregates.StdDev)
        if isinstance(expression, bad_aggregates):
            try:
                output_field = expression.input_field.output_field
                if isinstance(output_field, bad_fields):
                    raise NotImplementedError(
                        'You cannot use Sum, Avg, StdDev and Variance aggregations '
                        'on date/time fields in sqlite3 '
                        'since date/time is saved as text.')
            except FieldError:
                # not every sub-expression has an output_field which is fine to
                # ignore
                pass