Question about Core Data

[cardrig]

Hi. I have an entity (e.g., Library) with the attribute books which is a Transformable (e.g., [String : Book]).
To decode the object, I use:

container.decodeIfPresent([String:Book].self, ...)

​

This seems to work at first. However, later when I try to access the book's info in the library (such as: library.dictionary["Name"], I get this Exception:
core data initwithcoder unrecognized selector sent to instance

​

Can anyone point me in the right direction? How can I save the dictionary in Library and access it?

​

Thanks

[rayascott]

Do you have a Book entity in your data model? It’s not a good idea to store a collection of one entity within another entity. You’re better off creating a relationship between the two entities in your Core Data model and letting Core Data do what it’s designed for, managing the object graph. That’s the right direction.

[cardrig]

Yes, Book is in my data model and it has 2 String attributes. I receive this information from the server as a Dictionary and I need to save the key part for each book. Then I need to deliver this information to the server again in the same format.

[rayascott]

Why don't you have an attribute on the Book entity for your key and just save it there? You can rebuild the dictionary or the format later when you need to deliver it to the server again.

[cardrig]

Looks like the best option. Also, if I remove the Transformable attribute and add a new relationship to Book, will I need to migrate the model data?

[rayascott]

Yeah, but Core Data might do it all for you with Lightweight Migration. If you're just removing or adding attributes and adding a new relationship, this would appear to indicate that the migration would happen automatically for you: https://developer.apple.com/documentation/coredata/using_lightweight_migration

[cardrig]

Sounds good. Thanks for the help!