Question

I have a model, where I have an entity called “Category”. This Category has a one-to-many relationship called “Products”. How do I set up the fetch request to only get the Category entities WITHOUT the products?

Answer 1

If you query against Category, product elements are loaded as faults. This is the default behavior of CoreData.

On the contrary if you use - (void)setRelationshipKeyPathsForPrefetching:(NSArray *)keys, you can load (pre-fetch) products when the request is executed.

From Core Data App

Faulting reduces the amount of memory your application consumes. A
fault is a placeholder object that represents a managed object that
has not yet been fully realized, or a collection object that
represents a relationship:

A managed object fault is an instance of the appropriate class, but
its persistent variables are not yet initialized. A relationship fault
is a subclass of the collection class that represents the
relationship. Faulting allows Core Data to put boundaries on the
object graph. Because a fault is not realized, a managed object fault
consumes less memory, and managed objects related to a fault are not
required to be represented in memory at all.

and

Pre-fetching is in effect a special case of
batch-faulting, performed immediately after another fetch. The idea
behind pre-fetching is the anticipation of future needs. When you
fetch some objects, sometimes you know that soon after you will also
need related objects which may be represented by faults. To avoid the
inefficiency of individual faults firing, you can pre-fetch the
objects at the destination.

Edit

If you need to count the number of products for a specific category, use - (NSUInteger)countForFetchRequest:(NSFetchRequest *)request error:(NSError **)error using a request like the following:

NSFetchRequest* request = // set up a request for Products
[request setPredicate:[NSPredicate predicateWithFormat:@"toCategory == %@", currentCategory]];
// count for fetch request here...

where toCategory is the inverse relationship from Products to Category and currentCategory is the category you have.