Swifty Tip:Resolving SwiftData Predicate Issue

While building an app using SwiftData, you might run into a strange error when using predicates with UUID values. This issue often looks like this:

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

This error usually occurs when using a predicate like this:

let fetchDescriptor = FetchDescriptor<TodoModel>(
    predicate: #Predicate { $0.id == todo.id }
)

Even though your code editor might not show any errors, the build fails due to macro transformations in SwiftData predicates. Fortunately, there’s a simple workaround for this issue!

The Fix: Use a Local Variable for the UUID

Instead of directly using todo.id inside the predicate, extract todo.id into a local variable and use that in the predicate:

delete: { todo in
    @Dependency(\.contextProvider.context) var databaseContext

    let context = try databaseContext()
    let todoId = todo.id // Create a local variable

    let fetchDescriptor = FetchDescriptor<TodoModel>(
        predicate: #Predicate { $0.id == todoId } // Use the local variable
    )

    if let todoModel = try context.fetch(fetchDescriptor).first {
        context.delete(todoModel)
    }

    do {
        try context.save()
    } catch {
        print("Failed to save context after deleting todo: \(error)")
        throw error
    }
}

Why Does This Happen?

The issue occurs because of the way SwiftData’s macros transform code during compilation. When using complex expressions like todo.id directly inside a predicate, the compiler might not be able to resolve it correctly. However, using a local variable simplifies the expression and makes it easier for the compiler to handle.

A Minimal Example

Let’s use a simple Todo app model to demonstrate the fix. Suppose you have a TodoModel in your app:

import SwiftData

@Model
class TodoModel {
    var id: UUID
    var title: String

    init(id: UUID = UUID(), title: String) {
        self.id = id
        self.title = title
    }
}

Now, let’s say you want to delete a specific TodoModel using its id:

delete: { todo in
    @Dependency(\.contextProvider.context) var databaseContext

    let context = try databaseContext()
    let todoId = todo.id // Local variable

    let fetchDescriptor = FetchDescriptor<TodoModel>(
        predicate: #Predicate { $0.id == todoId } // Use the local variable
    )

    if let todoModel = try context.fetch(fetchDescriptor).first {
        context.delete(todoModel)
    }

    do {
        try context.save()
    } catch {
        print("Failed to save context after deleting todo: \(error)")
        throw error
    }
}

By using this approach, you can avoid the cryptic predicate expression errors and have a smoother development experience when working with SwiftData!

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This small but effective fix can save a lot of debugging time and make sure your app builds properly when dealing with SwiftData predicates.