SwiftData无法获取一对多反向关联数据问题求助

问题现象 #

创建三级关联的SwiftData模型并保存后，从子模型向上查询父节点正常，但查询父模型的子关联数据时返回空数组，不符合预期。

模型定义 #

import SwiftData

@Model
final public class FirstModel {
    let name: String

    @Relationship(deleteRule: .cascade, inverse: \SecondModel.parent) var children = [SecondModel]()

    init(name: String) {
        self.name = name
    }
}

@Model
final public class SecondModel {
    let parent: FirstModel
    let name: String

    @Relationship(deleteRule: .cascade, inverse: \ThirdModel.parent) var children = [ThirdModel]()

    init(name: String, parent: FirstModel) {
        self.name = name
        self.parent = parent
    }
}

@Model
final public class ThirdModel {
    let parent: SecondModel
    let name: String

    init(name: String, parent: SecondModel) {
        self.name = name
        self.parent = parent
    }
}

实例创建与保存代码 #

let schema = Schema([
    FirstModel.self,
    SecondModel.self,
    ThirdModel.self
])

let container = try ModelContainer(for: schema)
let context = ModelContext(container)

let firstModel = FirstModel(name: "my first model")
let secondModel = SecondModel(name: "my second model", parent: firstModel)
let thirdModel = ThirdModel(name: "my third model", parent: secondModel)

context.insert(firstModel)
context.insert(secondModel)
context.insert(thirdModel)

try context.save()

查询代码 #

print("-- Fetch Third Model")
let thirdFetchDescriptor: FetchDescriptor<ThirdModel> = FetchDescriptor<ThirdModel>(predicate: #Predicate { $0.name == "my third model" })
let thirdModels = try context.fetch(thirdFetchDescriptor)
for entry in thirdModels {
    print(">>> \(entry) - \(entry.parent) - \(entry.parent.parent)")
}

print("-- Fetch First Model")
let firstFetchDescriptor: FetchDescriptor<FirstModel> = FetchDescriptor<FirstModel>(predicate: #Predicate { $0.name == "my first model" })
let firstModels = try context.fetch(firstFetchDescriptor)
for entry in firstModels {
    print(">>> \(entry) - \(entry.children)")
    for child in entry.children {
        print("\t>>> \(child) - \(child.children)")
    }
}

实际输出结果 #

-- Fetch Third Model
>>> ThirdModel - SecondModel - FirstModel

-- Fetch First Model
>>> FirstModel - []

预期输出结果 #

-- Fetch First Model
>>> FirstModel - [SecondModel]
    >>> SecondModel - [ThirdModel]

问题原因与解决方案 #

问题核心是未维护关联关系的双向绑定：创建子模型时仅设置了parent属性，但未将子模型添加到父模型的children数组中。SwiftData不会自动同步反向关联，必须手动处理。

方案1：手动维护反向关联 #

修改实例创建代码，在创建子模型后将其加入父模型的children数组：

let firstModel = FirstModel(name: "my first model")
let secondModel = SecondModel(name: "my second model", parent: firstModel)
// 手动将子模型加入父模型的关联数组
firstModel.children.append(secondModel)

let thirdModel = ThirdModel(name: "my third model", parent: secondModel)
// 手动将子模型加入父模型的关联数组
secondModel.children.append(thirdModel)

context.insert(firstModel)
try context.save()

方案2：优化模型初始化方法（推荐） #

在子模型的初始化方法中自动维护反向关联，避免手动操作遗漏：

// 修改SecondModel的初始化
@Model
final public class SecondModel {
    let parent: FirstModel
    let name: String

    @Relationship(deleteRule: .cascade, inverse: \ThirdModel.parent) var children = [ThirdModel]()

    init(name: String, parent: FirstModel) {
        self.name = name
        self.parent = parent
        // 自动加入父模型的children数组
        parent.children.append(self)
    }
}

// 修改ThirdModel的初始化
@Model
final public class ThirdModel {
    let parent: SecondModel
    let name: String

    init(name: String, parent: SecondModel) {
        self.name = name
        self.parent = parent
        // 自动加入父模型的children数组
        parent.children.append(self)
    }
}

之后创建实例时无需手动调用append，反向关联会自动维护。

补充说明 #

SwiftData的@Relationship仅声明关联关系的存在，不会自动同步双向数据。必须确保关联的两端都被正确设置，才能在查询时获取到完整的关联数据。

内容的提问来源于stack exchange，提问作者OlivierM