Hello, I am having the same problem.

The thing is that I am trying to update values for other fields of the already existing "codes" (actually I'm doing this at the stock.production.lot model, which I have extended with inheritance and custom fields).

I don't know why the import doesn't let me overwrite some info on existing rows. I've done it on other models before. Maybe it's because of the way they were created.

Any advice?

Please start a new question and provide a link to the file you are importing and other details such as which fields and constraints you have added. It is hard to guess. More information will help.