get max for queryset.

    qs: queryset
    field: The field name to max.

def get_max(qs, field):
    """
    get max for queryset.

    qs: queryset
    field: The field name to max.
    """
    max_field = '%s__max' % field
    num = qs.aggregate(Max(field))[max_field]
    return num if num else 0