With a short example how to use that would be so nice for the newbies like me. Thanks! I need the stored date to be nil until it is defined.

Thanks Claude, as mentionned in the post below, problem is on deleting old data. Once data is saved in backup file, I used a batchDeleteRequest to flush all data on the entity where I replaced the attribute by a relationship. But I got runtime exception on the contentView, where that entity is listed (the code of the batchDeleteRequest being in a child view of that contentView). Honnestly I did not try further. I found it was easier to delete and reinstall the app.

Thanks Darkpaw would you have a link to a tutorial for a proper data migration ? Because I got your idea but have difficulties to know what is exactly meant by 'new version of the model' (does it mean a new data model within the app with a different name?), 'mapping model' and I have no idea how to tell Core Data to do such operation. Sorry.

Thanks for your interest in my post. I am trying to attach a zip of the project but it seems I cannot edit my post anymore and there is no attachement option to this reply. I tried to create a GitHub repository but as it is my first time I am not sure it is ok. Here is the link: GraphOds.zip

As for the steps to reproduce the issue, just run the app. The txt file contains about 2000 words (the full dictionnaire in french has 170 000 entries). The console displays the progress of each step. At the end you have a list view showing: on the left a Draw, in the middle the Link letter and on the right the augmented Draw,i.e. : Draw / Letter / Draw(Draw + Letter). And obviously, many Draws on the right are missing. That is the point !

Got it ! Actually Draw.minus must be a to-many relationship, indeed: one draw can result from adding one letter to more than one shorter draw. Thanks a lot.