关系可选时的 SwiftData 谓词

Question

考虑 SwiftData 中的以下关系：

@Model
class Category: Decodable, Identifiable {
    var id: Int
    var title: String
    @Relationship var questions = [Question]() 
}

 @Model
 class Question: Decodable, Identifiable {
    var id: Int
    @Relationship var category: Category?
 }

当我在保存问题后尝试从类别中获取所有问题时，就会出现问题：

let catId = 7
let categoryPredicate = #Predicate<Category> { $0.id == catId }
let categoryDescriptor = FetchDescriptor(predicate: categoryPredicate)
let categories = try modelContext.fetch(categoryDescriptor)
if let cat = categories.first {
    cat.questions = parsedQuestions
}

//now try to fetch the questions out of the modelContext so they can be used and updated 
let questionPredicate = #Predicate<Question> { $0.category.id == catId } //THIS LINE THROWS THE ERRORS
let questionDescriptor = FetchDescriptor(predicate: questionPredicate)
            
do {
    questions = try modelContext.fetch(questionDescriptor)
} catch {
    fatalError("unable to find any questions for \(category.title)")
}

我收到这组错误：

无法推断通用参数“ID”
在“可选”上引用实例方法“id”要求“类别”符合“视图”
类型 '(ID) -> some View' 不能符合 'BinaryInteger'

在这种情况下我怎样才能编写一个正确的谓词而不出现错误？

Answer 1

由于您的关系是可选的，因此您需要在谓词中使用可选

let questionPredicate = #Predicate<Question> { $0.category?.id == catId }