Model Key Path with Core Data and Bindings in Xcode 4

Question

I am having two objects in my model: Person and Address. Person has a key name and a relationship to Address and Address has a key street.

I have designed the user interface to show a table with one column (Person.name) and two text fields with name and street.

I have created one NSArrayController with managedObjectContext set and ObjectController set to Entity and EntityName Person.

Running the app the table shows all added persons. The two text fields are connected to the Person-ArrayController via Bindings tab:

nameField with value: Bind To: Person / Controller Key: selected / Model Key Path: name

streetField with value: Bind To: Person / Controller Key: selected / Model Key Path: address.street

I am able to write something into name and it is stored persistently. I am also able to enter something into streetField but it is not stored.

Does anyone know why??

Not enough information to solve this. The way you described it should be working. — Mundi
Thats what I assumed too. But try it out in a scratch project and you will see it does not work. — maxbareis

Rakesh Rakesh · Accepted Answer · 2012-08-15T13:28:58

When the array controller creates the person object, something similar to below code happens:
(assume moc is the AppDelegate managedObjectContext,also assume Person is the sub-class of NSManangedObject):

Person *personObj = [NSEntityDescription insertNewObjectForEntityForName:@"Person" inManagedObjectContext:moc]; 
personObj.name = @"something"; //some dummy values
[personObj.address setValue:@"dummy address" forKey:@"street"];

If personObj.address is logged , nil will be printed on the console. This is because the personObj.address is an Address object which till now has not been instantiated. Consequently personObj.address.street will be NULL. This is what happens in your case also.

if you change the above code to the following it will work fine.

Person *personObj = [NSEntityDescription insertNewObjectForEntityForName:@"Person" inManagedObjectContext:moc];

//create an Address object and assign it to personObj.address (setting the relationship)
personObj.address= [NSEntityDescription insertNewObjectForEntityForName:@"Address" inManagedObjectContext:moc]; 
personObj.name = @"something"; //some dummy values
[personObj.address setValue:@"dummy address" forKey:@"street"];

and this can't be done using binding.

In short streetField bound to address.street doesn't get any value because address doesn't point to an instantiated object.

Model Key Path with Core Data and Bindings in Xcode 4

1 Answers