CODEWITHSUNDEEP
objective-ccore-data
aahrens
aahrens

Reputation: 5590

CoreData Fetching Results With 3 Entities and Many to Many Relationships

I have a DataModel that has a Category, SubCategory, and DetailedItem entities. Category has a 1-to-many to the SubCategory. Each SubCategory has a 1-to-many relationship to DetailedItem. So a Category has a Set of subCategories and for each SubCategory it has a Set of detailedItems.

Category has a Set property of subCategories, and a Name property. SubCategory has a Set property of detailedItems, a Name Property, and Category parent property

My question is how can I use NSFetchedResultsController to retrieve all the detailedItems from a given SubCategory in a given Category.

I'm trying something like this but I think my NSPredicate is incorrect

 if (__fetchedResultsController == nil) {
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entityDescription = [NSEntityDescription entityForName:@"DetailedItem" inManagedObjectContext:[self managedObjectContext]];
    [fetchRequest setEntity:entityDescription];

     NSPredicate *predicate = [NSPredicate predicateWithFormat:@"Category.name CONTAINS[c] 'Movies'"];

    [fetchRequest setPredicate:predicate];


    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:YES]; // probably not needed
    [fetchRequest setSortDescriptors:[NSArray arrayWithObject:sortDescriptor]];

    NSFetchedResultsController *fetchedResults = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:[self managedObjectContext] sectionNameKeyPath:nil cacheName:@"AllItems"];
    fetchedResults.delegate = self;
    __fetchedResultsController = fetchedResults;

    NSError *error;
    if (![__fetchedResultsController performFetch:&error]) {
        NSLog(@"Error fetching");
    }

    [sortDescriptor release];
    [fetchRequest release];
}

The Goal being if Category == Food and SubCategory == Meat, then i'm getting a list of all detailedItems that fall under that Category & SubCategory. If anything isn't clear I'll try to provide more context.

Upvotes: 0

Views: 488

Answers (1)

TechZen
TechZen

Reputation: 64428

If your datamodel looks like:

Category{
  name:string
  subCategories<-->>SubCategory.category
}

SubCategory{
  name:string
  category<<-->Category.subCategories
  detailedItems<-->>DetailItem.subCategory
}

DetailedItem{
  name:string
  subCategory<<-->SubCategory.detailedItems
}

... and you want a table of DetailedItem objects, you set the fetch entity to DetailedItem and then you have all your keypaths begin from the DetailItem entity.

If you just have the names of the Category and SubCategory objects then you could connstruct a predicate like:

 NSPredicate *p = [NSPredicate predicateWithFormat:@"subCategory.name==%@ AND subCategory.category.name==%@", subCatName,catName];

Note that if you have duplicate name values in either entity's objects, you will get all matching objects.

If you have the actual Category and SubCategory objects in hand you can do a faster test with:

 NSPredicate *p = [NSPredicate predicateWithFormat:@"subCategory==%@ AND subCategory.category==%@", aSubCatObj,aCatObj];

Upvotes: 1

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