iOS Core Data问题：重复ID对象保存未覆盖旧对象

我是Core Data新手，保存对象时碰到个问题：希望保存相同ID（主键）的对象时，不会生成新条目，就像Android Room里的OnConflictStrategy.REPLACE那样（二者都基于SQLite）。我已经用了合并策略但没效果，相关代码和数据模型如下：

保存方法代码 #

static func addPost(post:Post){
    guard let context = context else {
        print("post add error with context core data")
        return
    }
    context.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy
    let p = PostDao(context: context)
    p.id = post.id
    p.username = post.username
    p.title = post.title
    p.photo = post.photo
    p.location = post.location
    p.contactPhone = post.contactPhone
    p.isPostDeleted = post.isPostDeleted!
    p.postDescription = post.postDescription
    p.lastUpdated = post.lastUpdated
    p.contactEmail = post.contactEmail
    
    do{
        try context.save()
    }catch let error as NSError{
        print("post add error core data\(error) \(error.userInfo)")
    }
}

Post 实体类代码 #

class Post{
    
    public var id: String? = ""
    public var title: String? = ""
    public var username: String? = ""
    public var postDescription: String? = ""
    public var photo: String? = ""
    public var location: String? = ""
    public var contactPhone: String? = ""
    public var contactEmail: String? = ""
    public var isPostDeleted: Bool? = false
    public var lastUpdated:Int64 = 0
    
    init(){}
    
    init(post:PostDao){
        id = post.id
        title = post.title
        postDescription = post.postDescription
        username = post.username
        photo = post.photo
        isPostDeleted = post.isPostDeleted
        lastUpdated = post.lastUpdated
        location = post.location
        contactPhone = post.contactPhone
        contactEmail = post.contactEmail
    }
    
}

PostDao Core Data 扩展代码 #

extension PostDao {

    @nonobjc public class func fetchRequest() -> NSFetchRequest<PostDao> {
        return NSFetchRequest<PostDao>(entityName: "PostDao")
    }

    @NSManaged public var id: String?
    @NSManaged public var title: String?
    @NSManaged public var username: String?
    @NSManaged public var photo: String?
    @NSManaged public var postDescription: String?
    @NSManaged public var location: String?
    @NSManaged public var contactPhone: String?
    @NSManaged public var contactEmail: String?
    @NSManaged public var isPostDeleted: Bool
    @NSManaged public var lastUpdated: Int64

}

extension PostDao : Identifiable {

}

数据模型 #

解决方法 #

你的问题核心在于每次调用addPost都直接创建了新的PostDao对象，没有先检查数据库中是否存在相同ID的条目，合并策略只有在上下文出现对象冲突时才会生效，而你当前的逻辑根本没触发冲突。

正确的实现步骤应该是：

1. 先根据传入Post的ID查询数据库中是否已有对应的PostDao对象
2. 如果存在，就更新该对象的属性；如果不存在，再创建新对象
3. 合并策略可提前设置到上下文，无需每次保存时重复设置

修改后的addPost方法示例：

static func addPost(post: Post) {
    guard let context = context else {
        print("post add error with context core data")
        return
    }
    // 建议将合并策略放到上下文初始化代码中，无需每次调用都设置
    context.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy
    
    // 1. 根据ID查询已有对象
    let fetchRequest: NSFetchRequest<PostDao> = PostDao.fetchRequest()
    fetchRequest.predicate = NSPredicate(format: "id == %@", post.id ?? "")
    
    do {
        let existingPosts = try context.fetch(fetchRequest)
        let postDao: PostDao
        
        if let existingPost = existingPosts.first {
            // 2. 存在则更新属性
            postDao = existingPost
            postDao.username = post.username
            postDao.title = post.title
            postDao.photo = post.photo
            postDao.location = post.location
            postDao.contactPhone = post.contactPhone
            postDao.isPostDeleted = post.isPostDeleted ?? false
            postDao.postDescription = post.postDescription
            postDao.lastUpdated = post.lastUpdated
            postDao.contactEmail = post.contactEmail
        } else {
            // 3. 不存在则创建新对象
            postDao = PostDao(context: context)
            postDao.id = post.id
            postDao.username = post.username
            postDao.title = post.title
            postDao.photo = post.photo
            postDao.location = post.location
            postDao.contactPhone = post.contactPhone
            postDao.isPostDeleted = post.isPostDeleted ?? false
            postDao.postDescription = post.postDescription
            postDao.lastUpdated = post.lastUpdated
            postDao.contactEmail = post.contactEmail
        }
        
        // 保存上下文
        try context.save()
    } catch let error as NSError {
        print("post add error core data \(error) \(error.userInfo)")
    }
}

另外还有两个优化建议：

• 给id字段添加唯一约束：在Core Data模型编辑器中，选中PostDao实体的id属性，在右侧检查器的「Constraints」里添加id UNIQUE约束，从数据库层面阻止重复ID插入，配合合并策略更可靠
• 替换强制解包post.isPostDeleted!：改用post.isPostDeleted ?? false的可选绑定方式，避免因nil值导致崩溃

内容的提问来源于stack exchange，提问作者Shimon Cohen