Core data refuses to work

Question

hi.
I have a very simple model, it contains 1 entity with 4 attributes.
I have created an ObjectController in IB, and bound it to my persistent document's object context.
I follow this code (found in the core data documentation)to get a reference to the Object, and to set/get values:
NSManagedObject *myDoc = [NSEntityDescription insertNewObjectForEntityForName:@"BKSketchDocMO" inManagedObjectContext:[dataSource managedObjectContext]];
thePanX = [[myDoc valueForKey:@"docPanX"] floatValue];
[myDoc setValue:[NSNumber numberWithFloat:[self bounds].size.width/2] forKey:@"docPanX"];
now we come to the problem.
its doesn't work. theres an entity. I can get a reference to it, and it ALWAYS has the default values. NOTHING i do changes the values, NOTHING i do gives me anything Except the default values.
and since core data is basically a black box... I CAN"T DEBUG IT.
can anyone, shed any light on anything that might explain why I can get the object, get values, but I cannot set values?
signed,
getting frustrated by the core data double-talk and general lack of any real, and useful documentation.
Message was edited by: Edward Devlin1

Answer 1

Hi kids, yet again I have answered my own question. in my application, I could not make my object graph self populate, and then when i did get it to populate programatically, I could not access the data.
I have solved both problems. one was a conceptual problem, and the other was operator error building off of a conceptual problem.
first, lets tackle why my object graph doesn't auto populate. in IB i made an Object controller, and I hooked it up to my entity. I told the controller to automatically prepare content. I had assumed that it meant that when my Object Context was instantiated, it would automatically create an instance of my referenced object. this is Not how core data works. Currently i am working under the impression that "automatically prepares content" mean that when an entity is instantiated, it will fill out the appropriate values, with the defaults you set up in your object model.
so core data Does not actually instantiate anything for you. once I made this conceptual leap, I was still hazy on the implementation. I tried adding new entities upon initialization, but that tended to over-write entities opened from a saved document. in the end, I simply settled on making sure the entity existed when i tried to access its information. If it didn't exist I made one before trying to access it. in practice, this is a very slick and robust solution.
now for problem #2... every time I tried to access my data, it is like it was zeroed out to the default settings.
well, thats exactly what was happening. my entity was valid, I was making changes to it, and I was still getting the default values back. How was this happening? I didn't understand what i was doing.
heres what I thought was happening. I thought I was making a new ManagedObject, and filling it with the values of the entity I was trying to access, then I got the values from that ManagedObject, and discarded it.
What was really happening: If you look at my code, you'll see a method called : "insertNewObjectForEntityForName:" this is the method I THOUGHT was going to my ObjectContext, getting my entity and creating a ManagedObject from it for me to interface with. Clever coders will see the problem right away. Insert new Object? I was creating a new instance of my entity each time i Thought I was accessing the original. holy crap! of course I was only getting the default values... and sure I was able to make a change to the values, but the next time I tried to access them, I would just get a new entity, filled with default values!
so InsertNewObject, was the wrong approach... what turned out to be the correct approach? heres why i was having such a bad time of this... NSDictionary, is roughly analogous to Core Data. I have alot of experience w/ NSDictionary, and so i expected a straight forward... give me an object based on this key method. I understood that i was going to have to work through a proxy, but I had assumed a little too much about how Core data works. its not like NSDictionary at all. it works much more like a search engine... you tell it where, and what to look for and it will bring you back a list of things that are kinda like what you're looking for.
This is called a fetch request. Its a much more elaborate way to look for stuff than a simple objectForKey: method. its how you get your information from core data. Information comes back as an array of NSManagedObjects. The documentation made everything so much more difficult because it spoke of fetched values as something extra or added on to core data. Kids say it along w/ me.... fetch requests are how you get info from core data. repeat.