How to define a Fetched Property in an NSManagedObject Subclass

Question

In a project I am working on, we have multiple persistent stores and fetched properties are defined on the Entities to provide access to the objects that live in the different stores.

When I run Editor -> Create NSManagedObject Subclass, the fetched properties do not get populated in the subclass, and therefore are not accessible in the different controllers that use this Entity.

My curiosity is how to define these objects in the subclass so that they can be used.

For example imagine I have some object below called "Some Object", and this object has a fetched property on it called "imageFile" (The File object lives in a different store so cannot be referenced directly)

class SomeObject: NSManagedObject {

   @NSManaged var name: String
   @NSManaged var id: String
   @NSManaged var imageID: String
   @NSManaged var imageFile: File             //Not generated automatically like the rest
}

Unfortunately the above attempt fails with the following error:

unrecognized selector sent to instance 0x60800865de50

So my question is a nutshell, is how do you access Fetched Properties, or what is the syntax to reference them.

Please no answers saying "Don't use Fetched Properties" or "Use only one persistent store". I already know how to use normal relationships and want to know how to utilize this feature of Core Data. Thanks in advance!

UPDATE

Trying some of the solution's posted below I ran into some interesting information that may help. I printed out the object using "po someObject" and was suprised to see the following in the output under the data attribute:

imageFile = "<relationship fault: 0x618000043930 'imageFile'>";
imageID = "some Id"

However when trying to access imageFile using someObject.imageFile I am unable to access it. Using the valueForKey["imageID"] I am able to get a reference, but it fails on the cast to File every time. When printing the object out I get:

Optional(Relationship fault for (<NSFetchedPropertyDescription: 0x6180000e1780>), name imageFile, isOptional 1, isTransient 1, entity SomeObject...

Final Update

the valueForKey["imageID"] will trigger the fault and fetch the property, I had the attributes flipped in my xcdatamodelid file, and thats why it wasn't finding it at first.

A fetched property is represented as an *array*, so you have to cast it to `[File]` . Then you can retrieve some (e.g. the first) element of that array. — Martin R, May 09 '15 at 21:29

score 3 · Accepted Answer · answered May 09 '15 at 21:09

3

In Objective-C you could define a @dynamic property file in a category on SomeObject, but something similar does not exist in Swift (as far as I know).

So the only possibility is to use Key-Value coding to retrieve the fetched property (which is always represented as an array):

if let files = yourObject.valueForKey("imageFile") as? [File] {
    // ...
}

Of course you can wrap this into a computed property as suggested in @gutenmorgenuhu's answer.

answered May 09 '15 at 21:09

Martin R

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This did it! I had the attributes flipped in the fetchedProperty and thats why it didn't access the first time. Thanks! – Unome May 09 '15 at 21:32

score 2 · Answer 2 · answered May 09 '15 at 20:33

2

If you want to add this to the same class as the NSManagedObject, you can use the extensions feature:

extension SomeObject{
  var imageFile: String {
    get {// Code to return your fetchedProperty
    }
  }
}

answered May 09 '15 at 20:33

gutenmorgenuhu

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I've got an extension for this NSManagedObject, but what would I put within the return value on the request? Do I need to write a fetch request to access this? – Unome May 09 '15 at 20:39
that depends where your value is stored and how it is retrieved. – gutenmorgenuhu May 09 '15 at 20:41
A fetched property has been defined in the Core Data xcdatamodelid file. The "File" entity is stored in a different persistent store than the "Some Object" entity. – Unome May 09 '15 at 20:45

How to define a Fetched Property in an NSManagedObject Subclass

2 Answers2