Swift Core Data按ID查询失败，按名称查询正常问题求助

正在进行单元测试，验证Core Data操作时遇到问题：数据库中存在一条FoodItem实体记录：

<EasyFPU.FoodItem: 0x6000021b00f0> (entity: FoodItem; id: 0x6000002a02c0 <x-coredata://55F63A9D-9195-4991-8B24-8A3FF6301B1E/FoodItem/p6>; data: {
    caloriesPer100g = 72;
    carbsPer100g = "10.4";
    category = Product;
    composedFoodItem = nil;
    favorite = 0;
    id = "220458AD-3216-45A2-9FC3-32285A2A36D0";
    ingredients =     (
    );
    name = Alpenzwerg;
    sugarsPer100g = "9.4";
    typicalAmounts =     (
    );
})

使用按名称查询的函数可以正常获取该记录：

static func getFoodItemByName(name: String) -> FoodItem? {
    let predicate = NSPredicate(format: "name == %@", name)
    let request: NSFetchRequest<FoodItem> = FoodItem.fetchRequest()
    request.predicate = predicate
    if let result = try? CoreDataStack.viewContext.fetch(request) {
        if !result.isEmpty {
            return result[0]
        }
    }
    return nil
}

但使用按ID查询的函数却返回空结果：

static func getFoodItemByID(_ id: String) -> FoodItem? {
    let predicate = NSPredicate(format: "id == %@", id)
    let request: NSFetchRequest<FoodItem> = FoodItem.fetchRequest()
    request.predicate = predicate
    if let result = try? CoreDataStack.viewContext.fetch(request) {
        if !result.isEmpty {
            return result[0]
        }
    }
    return nil
}

数据模型定义如下：

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<model type="com.apple.IDECoreDataModeler.DataModel" documentVersion="1.0" lastSavedToolsVersion="23231" systemVersion="24A335" minimumToolsVersion="Automatic" sourceLanguage="Swift" usedWithCloudKit="YES" userDefinedModelVersionIdentifier="">
    <entity name="FoodItem" representedClassName=".FoodItem" syncable="YES">
        <attribute name="caloriesPer100g" attributeType="Double" defaultValueString="0.0" usesScalarValueType="YES"/>
        <attribute name="carbsPer100g" attributeType="Double" defaultValueString="0.0" usesScalarValueType="YES"/>
        <attribute name="category" optional="YES" attributeType="String"/>
        <attribute name="favorite" attributeType="Boolean" defaultValueString="NO" usesScalarValueType="YES"/>
        <attribute name="id" optional="YES" attributeType="UUID" usesScalarValueType="NO"/>
        <attribute name="name" optional="YES" attributeType="String"/>
        <attribute name="sugarsPer100g" attributeType="Double" defaultValueString="0.0" usesScalarValueType="YES"/>
        <relationship name="composedFoodItem" optional="YES" maxCount="1" deletionRule="Nullify" destinationEntity="ComposedFoodItem" inverseName="foodItem" inverseEntity="ComposedFoodItem"/>
        <relationship name="ingredients" optional="YES" toMany="YES" deletionRule="Nullify" destinationEntity="Ingredient" inverseName="foodItem" inverseEntity="Ingredient"/>
        <relationship name="typicalAmounts" optional="YES" toMany="YES" deletionRule="Cascade" destinationEntity="TypicalAmount" inverseName="foodItem" inverseEntity="TypicalAmount"/>
    </entity>
</model>

两处查询代码逻辑一致，使用同一数据库，请问问题出在哪里？

问题核心是数据类型不匹配：

• 数据模型中id字段的类型是UUID，但查询时传入的是String类型的ID字符串，直接用字符串和UUID实例比较会完全不匹配，导致查询无结果。
• 而name字段本身就是String类型，所以按名称查询可以正常工作。

有两种修正方案可选：

1. 修改查询函数，将字符串转为UUID
   先把传入的字符串ID转换成UUID实例，再用于谓词查询：

   static func getFoodItemByID(_ id: String) -> FoodItem? {
       guard let uuid = UUID(uuidString: id) else {
           return nil // 处理无效UUID字符串的情况
       }
       let predicate = NSPredicate(format: "id == %@", uuid as CVarArg)
       let request: NSFetchRequest<FoodItem> = FoodItem.fetchRequest()
       request.predicate = predicate
       return try? CoreDataStack.viewContext.fetch(request).first
   }
   

2. 修改数据模型，将id字段类型改为String
   如果业务场景更适合用字符串存储ID，可以直接在Core Data模型编辑器中，把id的属性类型从UUID改为String，原查询代码无需修改即可正常运行。

注意：若选择修改数据模型，需做好已有数据的迁移处理，避免数据丢失。

内容的提问来源于stack exchange，提问作者Ulrich