Hi. Do you tested storing the field? Odoo needs stored field to do a sort.

I had a similar problem and I solved it by creating a second computed field and use this where required.

Example to get the idea:

@api.multi
def _compute_sorted_related_records(self):
    for record in self:
        record.sorted_related_records = record.related_records.sorted(
            key=lambda r: r.my_sort_field
        ).ids

sorted_related_records = fields.Many2many(
    'related.model',
    compute=_compute_sorted_related_records,
)

Ravi Gadhia

I need to sort activity_date_deadline field in CRM which is datetime type field and that field is not stored in database

You can add '_order' comment to sort record while listing out in list view. In that you can assign your field name and sorting method.