Need assistance regarding core data relationship concepts, Not asking for any code

Question

I am currently working on core data and newbie, fetching composite name and contact numbers from address book. I now can save data in to-one & to-many relationship using KVC and subclassing NSManagedObject. But still some concepts are not clear to me.

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In normal DB we save data in primary & foreign key way. ID, compositeName fields in ContactName table and ID, number, type fields in ContactNumber table. Relationships can be easily created on both table ID fields.

In core data its done like this using KVC:

1.        NSManagedObject *contactNameEntity = [NSEntityDescription insertNewObjectForEntityForName:@"ContactName" inManagedObjectContext:self.managedObjectContext];

2.                [contactNameEntity setValue:@"TheName" forKey:@"compositeName"]

3.                NSMutableSet *relationAsSet = [contactNameEntity mutableSetValueForKey:@"contactNameRelation"];

    //fetching multi values like contact mobile, iPhone, home, work, other numbers and iterating them
4.                ABMutableMultiValueRef multiValues = ABRecordCopyValue(ref, kABPersonPhoneProperty);
5.                        CFArrayRef mVArray = ABMultiValueCopyArrayOfAllValues(multiValues);
6.                        for(int i=0; i



Now with entity subclasses method #1:

1.        ContactName *contactNameEntity = [NSEntityDescription insertNewObjectForEntityForName:@"ContactName" inManagedObjectContext:self.managedObjectContext];
2.                [contactNameEntity setCompositeName:(__bridge id)(compositeName)];

3.    for(int i=0; i


Now with entity subclasses which is similar to KVC method #2:

1.    ContactName *contactNameEntity = [NSEntityDescription insertNewObjectForEntityForName:@"ContactName" inManagedObjectContext:self.managedObjectContext];
2.             [contactNameEntity setCompositeName:(__bridge id)(compositeName)];

3.    NSSet *set = [contactNameEntity contactNameRelation];
4.            NSMutableSet *mSet = [NSMutableSet setWithSet:set];
5.            set = nil;

6.    for(int i=0; i


Questions:


In KVC method line 3, why a normal NSSet cannot be created? 
Why relationship is present as NSSet, although in normal DB relationship is just a simple thing but seems like complex in core data. 
Am I using the subclass generated methods correctly or Am I using subclass generated methods in standard way? (asking to verify my concepts).
Why method #1 and method #2 differs? both are using subclasses.
Is @property (nonatomic, retain) NSSet *contactNameRelation; and NSMutableSet *relationAsSet = [contactNameEntity mutableSetValueForKey:@"contactNameRelation"]; return same object/value?


*. Please avoid any silly mistake.

*. Please tell the questions I am asking is unclear.

Edit 1

ContactName entity xcode generated subclass .h file:

@property (nonatomic, retain) NSString * number;
@property (nonatomic, retain) NSSet *contactNumbers;
@end

@interface ContactName (CoreDataGeneratedAccessors)

- (void)addContactNumbersObject:(ContactNumber *)value;
- (void)removeContactNumbersObject:(ContactNumber *)value;
- (void)addContactNumbers:(NSSet *)values;
- (void)removeContactNumbers:(NSSet *)values;


and this is .m file:

@implementation ContactName

@dynamic compositeName;
@dynamic contactNumbers;

@end


I want to know how to correctly use these methods, and thats why method 1 and 2 are two different approaches I posted above based on this subclass. Whats correct and whats not please help me out. 

Edit 2

this is what I get when I log relationAsSet

$0 = 0x0854c1c0 Relationship 'contactNameRelation' on managed object (0x835bae0)  (entity: ContactName; id: 0x835ba80  ; data: {
    compositeName = myname;
    contactNameRelation =     (
    );
}) with objects {(
)}


and this 

$0 = 0x0821b460 Relationship 'contactNameRelation' on managed object (0x823fe50)  (entity: ContactName; id: 0x823fea0  ; data: {
    compositeName = myname;
    contactNameRelation =     (
        "0x821be40 "
    );
}) with objects {(
     (entity: ContactNumber; id: 0x821be40  ; data: {
    contactNumberRelation = "0x823fea0 ";
    number = 123;
    type = iphone;
})
)}

Martin R · Accepted Answer

As @svena already said, your names for the relationships are a bit "unconventional" (and it seems that you partially changed them already in your added code). So I will assume the following model and relationship names:

Core Data Model

The value of a to-many relationship is a NSSet. For example

ContactName *aContact = ...;
NSSet *relatedNumbers = aContact.contactNumbers;
// or equivalently, using KVC:
relatedNumbers = [aContact valueForKey:@"contactNumbers"];
for (ContactNumber *contactNumber in relatedNumbers) {
    NSLog(@"%@", contactNumber.number);
}

So you use the value of the relationship (a NSSet in this case) to traverse the object graph and find related object. But you cannot modify this set (it is immutable).

Then you have questions about

NSMutableSet *mutableRelatedNumbers = [aContact mutableSetValueForKey:@"contactNumbers"];

which is a very special thing. It is a mutable set that is "tied" to the aContact object. That means if you modify this set (add or remove something) then you effectively modify the contactNumbers relationship of the aContact object.

So

ContactName *aContact = ...;
ContactNumber *aContactNumber1 = ...;
ContactNumber *aContactNumber2 = ...;

// (A)
NSMutableSet *mutableRelatedNumbers = [aContact mutableSetValueForKey:@"contactNumbers"];
[mutableRelatedNumbers addObject:aContactNumber1];
[mutableRelatedNumbers addObject:aContactNumber2];

(which is the corrected version of your first code block) is a complicated way of doing this

// (B)
[aContact addContactNumbersObject:aContactNumber1];
[aContact addContactNumbersObject:aContactNumber2];

(which is your method #1). Another equivalent method is (because of the inverse relationship)

// (C)
aContactNumber1.contactName = aContact;
aContactNumber2.contactName = aContact;

or

// (D)
NSMutableSet *mset = [NSMutableSet set];
[mset addObject:aContactNumber1];
[mset addObject:aContactNumber2];
[aContact addContactNumbers:mset];

which is similar to (B), and is the corrected version of your method #2.

This are all valid methods to add an object to a relationship, but (C) is the easiest to use, followed by (B). (A) and (D) are rarely used. Another advantage of (C) is the better type checking.

So even if I did not answer the questions directly, I hope that this helps to clear things up!

Need assistance regarding core data relationship concepts, Not asking for any code

Answers (2)

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