thanks for your answer, but I would like to have that value for working with it. Is that possible?

When you saved record then title_from_a is display

Yes, but I want to display in a form before and not after saving. is that possible?

If you want to display as display name then it is not possible. if you want to display in form view then select a_id and after selecting a_id automatic display it's title in title_form_a field.

I just want to display in form view of class.b. but title_from_a does not appear on the form view.

Try to give _rec_name to class a.

For Ex:

class A(models.Model):

_name = 'class.a'

_rec_name = 'title'

title = fields.Char('Title')

b_ids = fields.One2Many('class.b', 'a_id')

it does not work. title_from_a is still empty. ;(