Question

I’m new to OS X programming but generally liking it. I have the following problem:

I have two core data entities linked through a one-to-many with their respective arraycontrollers (Stock Controller and Price History Controller, where the latter controller is bound to the Stock Controller, with Controller Key = selection and Model Key Path = priceHistory, which is the relationship that links the stock entity to the PriceDataPoint entity, controlled by the Price History Controller.

This all works like magic in my UI, where I can select stocks and add/remove price points to each one when it is selected. However, I need to be able to do this programmatically as well.

1. If I simply call [stockController add:self] the UI updates with new objects with the correct default values, linked to the selected stock — even though ‘self’ is not the correct class/entity. This is one point of confusion for me, which I don’t understand (I understand that the Stock Controller knows about the selected stock through the KVO binding and would likely set the relationship as required, but I don’t get how it ‘casts’ ‘self’, which is a fairly random class into the object type required (a plain NSManagedObject)? Secondly, if I do this, how do I get a reference back to that object so I can edit its values?

2. More importantly, however, if I then follow the Apple examples, create a new NSManagedObject through NSEntityDescription:insertNewObjectForEntity: and use [stockController addObject:Newly created Object] I can write the values I want before adding it but the relationship to the ‘parent’ stock is not set by the addObjects: method. I am sure I can figure out how to write this, too but with everything else in Cocoa being so elegant this just feels odd so I am hoping that someone here can clear this up very quickly and point me to an elegant way of doing it.

Answer 1

First, let’s dispel the magic by stating Core Data is built to work with the Cocoa Bindings mechanism (which gives you array controllers, among other things) and is built atop Key Value Coding / Key Value Observing mechanisms. The Core Data documentation clearly states you should have a good background in these technologies to understand how / why things are working.

For your first issue, you said you’re confused by the -add: method of NSArrayController. Have you looked at the signature/prototype? The argument (where you pass self) is (id)sender, which is the hallmark of a basic action (see “target/action” in the docs). You can pass self, some other object, or even nil if you want. The argument is NOT the object you wish to add to the collection managed by the array controller.

For your second issue, you’re on the right track if you want to customize things or keep a reference to the newly-inserted object. You can use the -addObject: method (which does take the object to add as its argument) but you’ll have to tell the array controller to -fetch: (another action w/sender as argument) in order to refresh the controller AND anything bound to its contents (such as a table/outline).