Predicate to NSPredicate conversion fails (not tagged as ObjC)

[helje5]

Follow up to #22 by @HealsCodes:

I've been playing around with Predicates + ManagedModels but always run into a dead-end seemingly because Model properties aren't exposed to the underlying Objective-C runtime so code like this doesn't work:

@Model
final class MyModel: NSManagedObject {
  var enabled: Bool

  convenience init() {
    self.enabled = false
  }
}

// the following works if MyModel is created using the traditional NSManagedObject / @NSManaged means

let p = #Predicate<MyModel> { $0.enabled == true } 
let predicate = NSPredicate(p) // returns nil, because apparently MyModel.enabled isn't bridged

Likewise a simple NSExpression(forKeyPath: \MyModel.enabled) fails with "Foundation/NSObject.swift:132: Fatal error: Could not extract a String from KeyPath \MyModel.enabled"

The same issue breaks interoperability with packages like PredicateKit which relies on being able to convert AnyKeyPath to String using ObjC bridging internals.

[helje5]

So this is currently generated for the property:

@Model
final class MyModel: NSManagedObject {
  @_PersistedProperty
  var enabled: Bool
  {
    set {
        setValue(forKey: "enabled", to: newValue)
    }
    get {
        getValue(forKey: "enabled")
    }
  }
}

And indeed, this is a Swift only property. Looks like just adding @objc will make this work:

@Model
final class MyModel: NSManagedObject {
  
  @objc var enabled2: Bool {
      set {
          setValue(forKey: "enabled", to: newValue)
      }
      get {
          getValue(forKey: "enabled")
      }
  }
}

let p = #Predicate<MyModel> { $0.enabled2 == true }
let predicate = NSPredicate(p) // works

print("Predicate:", predicate)
// Predicate: Optional(enabled2 == 1)

[helje5]

This turns out to be more difficult than expected :-) I.e. this is problematic:

    enum AddressType: Int {
        case home, work
    }
    @Model
    final class Address /*test*/ : NSManagedObject {
      var street     : String
      var type       : AddressType
      var person     : Person

We cannot add @objc to type, because:

Property cannot be marked @objc because its type cannot be represented in Objective-C

Not quite sure how we can figure out whether a node can be represented in ObjC 🤔

[helje5]

I added something to 0.8.14, but I don't think we can fully determine the ObjC convertibility in a macro, as that can't resolve types.

[HealsCodes]

Thank you for putting in all the effort! I'll gladly give 0.8.14 a go once I have time to sit down and work on my own project next.

For me it is already a huge bonus to be able to use key paths this way with types that can be mapped cleanly to Foundation and thus ObjC types.

I had good success manually adding the extra @obj to base types and using that in queries even with PredikateKit 😄