does anyone have a working example of a fetched-property in core-data?

Question

I have tried to use fetched properties a couple of times, and although it seems to be the right approach, it never works.

In my latest attempt I added the fetched-property to my entity, selected the other entity in the model as the 'destination', and set the predicate to a condition that I know is valid.

Problem 1: When the NSManagedObject-class for the entity is generated it does not include anything for the fetched-property. After some searching I added the declaration for it in the .h file and the @dynamic statement for it in the .m file (yes, I know it's an NSArray * type).

Problem 2: Even after that, when I access this property in code I get an exception being thrown that states something to the effect that the fetch-request does not have an entity. I am assuming that the 'entity' would be the one specified as the 'destination' and it is, in fact, there.

So, I'd like someone to provide a concrete working example (iOS platform) where a fetched-property is defined in the model, declared in a NSManagedObject-derived class, and actually used from code.

At this point I am giving up on this time-waster and simply implementing the fetch-request code myself.

Solution 1

Here's my relevant bits of code (including bits you've already mentioned):

My example has a 'Card' object that has a 1->many relationship with a 'Stats' object. Each 'Stats' object has an 'outcome' that can be 1-4. My fetched property is a simple one to give my 'Card' object an array of 'Stats' objects that are of 'outcome'=1 only.

I wanted to use the fetched property so that I could easily get hold of 'Card' objects that had more than a certain number and kind of 'Stats' objects.

So, in the 'Card' object I put the Fetched Property 'statsOfTypeOne', with Destination set to 'Stats'.

In the predicate for this fetched property I put

(SELF.outcome=1) AND (SELF.card=$FETCH_SOURCE)

'SELF' is the 'stats' record, and $FETCH_SOURCE magically becomes the 'Card' object when executed.

As you did, I put the following in the .h and .m files for the 'Card' object:

@property (nonatomic, retain) NSArray *statsOfTypeOne;
@dynamic statsOfTypeOne;

Then in my code I used:

[self.managedObjectContext refreshObject:cardInstance mergeChanges:YES];
[cardInstance valueForKey:@"statsOfTypeOne"]

to get at the array (although cardInstance.statsOfTypeOne should be fine). Without the refresh object it wasn't updating the Fetched property (as per the manual).

I think that's everything that I did to make it work. Let me know if it works for you.

Peter

Solution 2

Adding to @Peter's answer. Here's how I got it working in Swift 2.0 and Xcode 7:

import Foundation
import CoreData

@objc(Card)
class Card: NSManagedObject {

    @NSManaged var statsOfTypeOne: [Stat]

}

And then, to read the fetched property:

managedObjectContext.refreshObject(someCard, mergeChanges: true)
// This works and returns [Stat] type
someCard.statsOfTypeOne
// So does this
someCard.valueForkey("statsOfTypeOne") as! [Stat]

Solution 3

Have you taken a look at this previous question: Xcode 4 Core Data: How to use fetched property created in Data Model editor

Read through the accepted answer and all of the comments. It sounds like they have it sorted out.

Comments

[Sixten Otto]

Having the same problem: the entity is set in the model (and I can see that in the contents XML) but the compiled model that's loaded into the application doesn't have target entities set. Frustrating!

[Admin]

Thanks sosborn, but I've already been through that one -- that's where I found out the answer to Problem 1 in my post -- but using the property in the app throws the exception outlined in Problem 2.