Sorry but it doesnt work i have found nothing stored in the database :/

For old records I think you must update your module, you test it bay creating a new record?

That is what i have done exactly. But, i havnt found anything in the database

Hey friend in the example that you have put (account.invoice model) the field is depending from others thats why its stored in database

The field was created in the DB?

yes but the value is not displayed even store

I check if you have no dependency just add a required field to trigger it, or just add it on create methode

I mean @depends on a required field or just define it as a normal field and compute it in the create or write methode if there is no dependency on other fields

My field is calculaing the period from the create of the record till the date of the day. So that, it should be computed unless you have an other solution

So it depends on create_date you can add it as a field, unfortunately there no other solution, find this https://github.com/odoo/odoo/issues/9946

I have told you that !!!! My field should be recomputed but the create-date will not change so that my method will not be called

If I understand your need you can achieve it by adding a button to trigger the calculation you can't do it automatically