I am trying to import a set of data into a CoreData persistentStore. This is read-only data that will be presented to the user at run-time.
I have an entity called "Category" that has a one-to-many relationship with an entity called "Item", which in turn has an inverse relationship to a Category.
As I add Items to the context, how do I associate them with the correct Category? I can see in the SQLite dB that it is done by adding a Category field to the Item table, and probably uses the Categories primary key for the relationship. But the PK is behind-the-scenes... is there a method for making the connection?
I also see in my Category class that there are methods generated by CoreData for adding Items, but I am assuming that these are alos behind-the-scene methods that allow CoreData to maintain the relationships:
@interface Category (CoreDataGeneratedAccessors)
- (void)addItemObject:(Item *)value;
- (void)removeItemObject:(Item *)value;
- (void)addItems:(NSSet *)value;
- (void)removeItems:(NSSet *)value;
@end
I read in the programing guide that CoreData takes care of the other side of the relationship automatically, but I can't figure out how do do the initial link to a Category as I add Items.
Thanks
jk
Inverse relationships enable Core Data to propagate change in both directions when an instance of either the source or destination type changes. Every relationship must have an inverse. When creating relationships in the Graph editor, you add inverse relationships between entities in a single step.
There are different possibilities. If you already have a Category object (for instance obtained through a fetch request), and assuming the variables
Category *category;
Item *item;
then you simply do the following:
item.category = category;
or
[category setValue: category forKey:@"category"];
and you are done, since Core Data automatically sets the inverse relationships.
If you do not have a Category object, or you want to insert a new one, do the following:
// Create a new instance of the entity
Category *category = (Category *) [NSEntityDescription insertNewObjectForEntityForName:@"Category" inManagedObjectContext:managedObjectContext];
// add all of the category properties, then set the relationship
// for instance set the category name
[category setValue:@"myCategoryName" forKey:@"name"];
[category setValue:item forKey:@"item"];
Then you set this Category object for your Item object exactly as before. Finally, the methods you have shown are not used behind the scenes by Core Data: these methods are available to you, so that you can also do the following:
[category addItemObject:item];
or the inverse:
[item addCategoryObject:category];
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