CoreData compare two NSManagedObjects by their attributes and relationships - iphone

Question

I have filter object. It has several attributes and relationships. I want to create new filter object if there's no one with the same attributes & relationships. How to accomplish this ?

Answer 1

I would choose a more general approach..
I would take the NSEntityDescriptions of the object and build a predicate with all properties in that Description.
so like..
- (void)insertIfUnique:(NSManagedObject*)obj inContext:(NSManagedObjectContext*)ctx {
NSMutableString *format = [NSMutableString string];
NSEntityDescription *desc = obj.entity;
NSArray *attrs = desc.attributeKeys;
for(NSString *attr in attrs) {
if(format.length)
[format appendString:#" AND "];
[format appendFormat:#"%#==%#", attr, [obj valueForKey:attr]];
}
NSPredicate *p = [NSPredicate predicateWithFormat:format];
NSFetchRequest *f = [[NSFetchRequest alloc] initWithEntityName:desc.name];
f.predicate = p;
if([ctx countForFetchRequest:f error:nil]==0)
[ctx insertObject:obj];
}

Answer 2

You have to manually search CoreData for any existing filter objects. If not found, you can process to create a new filter:
Here is one helper function
+(id)uniqueEntityfForName:(NSString *)name
withValue:(id)value
forKey:(NSString *)key
inManagedObjectContext:(NSManagedObjectContext *)context {
NSFetchRequest *request = [[[NSFetchRequest alloc] init] autorelease];
request.entity = [NSEntityDescription entityForName:name inManagedObjectContext:context];
request.predicate = [NSPredicate predicateWithFormat:[key stringByAppendingString:#" == %#"], value];
NSArray *result = [context executeFetchRequest:request error:nil];
id entity = [result lastObject];
if (entity == nil) {
entity = [NSEntityDescription insertNewObjectForEntityForName:name inManagedObjectContext:context];
[entity setValue:value forKey:key];
} else {
entity = [result lastObject];
}
return entity;
}
I use this method like this:
SomeEntity *entity = [CDUtils uniqueEntityfForName:#"SomeEntity" withValue:#"foo" forKey:#"bar" inManagedObjectContext:context];
You may have to define your own predicate.