CoreData通过其属性和关系比较两个NSManagedObjects

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.