Core Data - save existing managed object and show it in an another view

Question

I'm working on an table drill-down style iPhone app that has prepopulated data. I use Core Data and NSFetchedResultsController to populate the table views. In the last level of the table view which shows an item (managed object) I want my user to be able to select that item which should eventualy be shown in another view. That other view would be a kind of a favorite list (implemented in a tab view). The user would then have a choice of deleting or adding other items to the favorite list.

My model has three entities each representing one level of table view. Higher level entity has a to-many relationship to lower level entity and inverse relationships are to-one

How do I use the existing managed object (object in the last level of table view) to save it and show it in favorite list view? Should I create new entity and establish relationship between the two?

Answer 1

Yes, you could create a new entity and store the relationship. It's not necessarily the only way to do it -- you could store pointers to your NSManagedObjects in a container like an NSMutableArray -- but if you want to remember that list for later (i.e. save it between launches), it might make the most sense to also store it using Core Data.

Answer 2

My model has three entities each representing one level of table view.

That is thinking backwards. What you have is three entities that exist logically in a hierarchy and the hierarchy of views reflects that logical structure. The views exist to display the data, the data does not exist to display the views. It's an important concept to grasp and if you fail to do so, your application design will always be overly complex, fragile and hard to extend and maintain. The data model always comes first and the logical relationships within the data model itself and the users interactions with that data ultimately control the UI structure of the app.

It's an easy trap to fall into because the instructional materials always start with the interface first. However, in real app design, you start with the data model first and work forward to the interface.

In this case, if you want to store favorites of some entity you have two choices. If the entity being a favorite is part of the core relationship between data and the user and you only have one set of favorites, then you could legitimately add a boolean "isFavorite" attribute to the entity and then just fetch al the entities where "isFavorite==YES"; If you have multiple list of favorites then the best method would to to create a FavoritesList entity and then relate each favorites entity to the objects it is supposed to list.

If the favorites are a minor and peripheral part of the user's interaction with the data you could store the objectIDs in the user defaults.