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NSPredicate
What if instead of retrieving every instance of an Entity I just need those instances with a specific attribute. For instance, if the ExampleEntity listed above had an attribute called state which is an Integer and I wanted to find all of the entities that had a state of zero, I would alter the code above as follows:
NSManagedObjectContext *context = [[NSApp delegate] managedObjectContext]; NSFetchRequest *request = [[[NSFetchRequest alloc] init] autorelease]; NSPredicate *predicate = [NSPredicate predicateWithFormat:@"state == 0"]; NSEntity *entity = [NSEntityDescription entityForName:@"ExampleEntity" inManagedObjectContext:context]; [request setEntity:entity]; [request setPredicate:predicate]; NSError *error; NSArray *result = [context executeFetchRequest:request error:"error]; NSAssert(result != nil, ([NSString stringWithFormat:@"Error fetching entity: %@", error]));
The only change from the previous example is the addition of a NSPredicate to the NSFetchRequest. The NSPredicate is similar in functionality to the where clause of a SQL query. It is possible to create compound predicates as well as simple ones. By utilizing predicates it is possible to retrieve exactly the entities needed by the application.