create relationship of new objects based on existing managed object property

Question

I have the following datamodel:

entityA<-->>entityB<<-->>entityC
entityD<-->>entityE

Note that entityD and entityE do not have relationship with A-C.

My DetailViewController takes in properties from managed objects and saves as a new managed object to the managedObjectContext.

I currently display the existing managed object in UILabels like this:

UILabel1.text = [self.selectedEntityB nameB];
UILabel2.text = [self.selectedEntityA nameA];

I then save to the context in the save action here:

- (IBAction)save:(id)sender {
    EntityD *newEntityD = [NSEntityDescription insertNewObjectForEntityForName:@"EntityD"
                                                      inManagedObjectContext:self.managedObjectContext];

    [newEntityD setValue:self.UILabel2.text forKey:@"nameD"];    

    EntityE *newEntityE = [NSEntityDescription insertNewObjectForEntityForName:@"EntityE"
                                                      inManagedObjectContext:self.managedObjectContext];

    [newEntityE setValue:self.UILabel1.text forKey:@"nameE"];

    NSError *anyError = nil;
    BOOL savedSuccessfully = [self.managedObjectContext save:&anyError];
    if (!savedSuccessfully) {

    }
}

This works for setting nameD and nameE. I'm trying create a relationship between the newly created objects for EntityD and EntityE. Lets say relationship from entityE to entityD is toD. I've tried in the save action:

  [newEntityE setValue:self.UILabel2.text forKey:@"toD"];

But I get error with: 'NSInvalidArgumentException', reason: '-[__NSCFString managedObjectContext]: unrecognized selector sent to instance 0x10939fd00'

How would I establish relationship between the newly created objects?

score 0 · Answer 1 · answered Jul 14 '14 at 20:03

I am assuming that you are using NSManagedObject subclasses. In this case I would advise to move away from the cumbersome setValue calls and instead use the convenient dot notation.

The relationship to toD points to an EntityD object, so it cannot be the string from the text field. Set the entityD instead:

newEntityE.toD = newEntityD;

score 0 · Accepted Answer · answered Jul 14 '14 at 20:10

The relationship is implicitly created when you assign EntityD to EntityE via the relationship 'property'. Take a look at the header files for each of these entities. You should have a property in EntityE like this (given the to-one relationship from E to D):

@property (nonatomic, retain) EntityD *toD;

This means that you have to assign an EntityD 'object' to newEntityE through it's relationship property toD as follows:

// assuming EntityD has some property like NSString *name then
newEntityD.name = self.UILabel2.text;
newEntityE.toD = newEntityD;

It looks like you are trying to assign an NSString directly instead of updating the name property of newEntityD and then assigning the newEntityD object to newEntityE through the relationship.

create relationship of new objects based on existing managed object property

2 Answers2