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I have a core data model (has made a simplifie to make it easier to explain what I want to do).
An author can published many books and a book can have many chapters. In the example I want to retreive all the Autors that has a chapter in a book where readDate is null.
- (NSArray *)incompleteChaptersInManagedObjectContext:(NSManagedObjectContext *)context
{
NSFetchRequest* fetch = [NSFetchRequest fetchRequestWithEntityName:NSStringFromClass([Author class])];
NSPredicate* uidCondition = [NSPredicate predicateWithFormat:@"books.chapters.readDate!=nil"];
[fetch setPredicate:uidCondition];
NSArray* resultArray = [context executeFetchRequest:fetch error:&error];
..
..
This does not work and I am struggling finding out how this can be done. At the moment I am declare an author mutablearray. - get all authors - loop through all their books - loop through all the chapters - if readDate is null add it to the author array
This works but I want to solve this without the nested loops.
204
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.
Fetched Properties in Core Data are properties that return an array value from a predicate. A fetched property predicate is a Core Data query that evaluates to an array of results.
Use Core Data to save your application's permanent data for offline use, to cache temporary data, and to add undo functionality to your app on a single device. To sync data across multiple devices in a single iCloud account, Core Data automatically mirrors your schema to a CloudKit container.
You initialize an instance of NSFetchRequest as generic type: NSFetchRequest<Venue> . At a minimum, you must specify a NSEntityDescription for the fetch request. In this case, the entity is Venue . You initialize an instance of NSEntityDescription and use it to set the fetch request's entity property.
For nested to-many relationships a SUBQUERY is necessary (as @geo already correctly said). But one SUBQUERY should be sufficient:
[NSPredicate predicateWithFormat:@"SUBQUERY(books, $b, ANY $b.chapters.readData == NULL).@count > 0"]
(I assume that "books" and "chapters" are really to-many relationships, even if your diagram show them as to-one relationship.)
170
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