SwiftData @Query with #Predicate on Relationship Model

Question

XCode 15 beta 6.

Just want to do a very simple Query Predicate where the relationship model matches:

@Query(
    filter: #Predicate<Piece> {
        $0.artist == selectedArtist
    },
    sort: [
        SortDescriptor(\Piece.age, order: .reverse)
    ]
) var pieces: [Piece]

and I'm receiving error:

Cannot convert value of type 'PredicateExpressions.Equal<PredicateExpressions.KeyPath<PredicateExpressions.Variable, Artist>, PredicateExpressions.Value>' to closure result type 'any StandardPredicateExpression'

I also tried .id and .persistentModelId on the artist but no luck.

This seems like the most basic predicate use case so I'd be shocked if it's not supported. Any ideas what i'm missing?

My models are basic:

@Model
final class Piece {
    var age: Int
    var artist: Artist
}

And

@Model
final class Artist {
    var name: String
    
    @Relationship(
        deleteRule: .cascade, 
        inverse: \Piece.artist
    )
    var pieces: [Piece]?
}

Answer 1

You gave me an idea and it worked !!!

Considering that you can't use another model in the predicate, then first set a variable with the persistentModelID and use that variable in the predicate.

I was having the same problem and this worked for me. Of course you need to set the query in your init()

EDIT: I added part of the code that could be helpful.

@Query private var homes: [Home]

init(session: Session) {
    
    let id = session.persistentModelID
    let predicate = #Predicate<Home> { home in
        home.session?.persistentModelID == id
    }
    
    _homes = Query(filter: predicate, sort: [SortDescriptor(\.timestamp)] )
}

Answer 2

Per answer here, it seems like #Predicate will not let you use another @Model in the code block. Based on this logic, I don't think there's a way to do this as part of @Query without jumping through a bunch of hoops.

If your dataset is relatively small, the best suggestion is to filter it yourself:

@Query(
    sort: [
        SortDescriptor(\Piece.age, order: .reverse)
    ]
) var pieces: [Piece]

private var filteredPieces: [Piece] {
    return pieces.compactMap { piece in
        guard let artist = piece.artist else {
            return nil
        }
        return artist == selectedArtist ? piece : nil
    }
}

and you can use your filtered data below:

var body: some View {
    List {
        ForEach(filteredPieces) { piece in
            //show filtered stuff
        }
    }
}