在领域迁移期间删除 List<T> 中的类答案

【问题标题】：Deleting Classes in List<T> during Realm Migration在领域迁移期间删除 List<T> 中的类
【发布时间】：2020-10-30 06:29:38
【问题描述】：

例如。我有旧模型，像这样：

class Foo:Object {
    @objc dynamic var id = ObjectId.generate()
    let bars = List<Bar>()
    
    override class func primaryKey() -> String? {
        return "id"
    }
}

class Bar:Object {
    @objc dynamic var id = ObjectId.generate()
    
    override class func primaryKey() -> String? {
        return "id"
    }
}

以及新型号：

class Foo:Object {
    @objc dynamic var id = ObjectId.generate()
    
    override class func primaryKey() -> String? {
        return "id"
    }
}

迁移代码：

let config = Realm.Configuration(
    schemaVersion: 1,
    migrationBlock: {migration, oldSchemaVersion in
        if oldSchemaVersion < 1 {
            migration.deleteData(forType: "Bar")
        }
    })
let realm = try! Realm(configuration: config)

运行时报错“Table is target of cross-table link columns”。

如果我先跑

let config = Realm.Configuration(
    schemaVersion: 1,
    migrationBlock: {migration, oldSchemaVersion in
        if oldSchemaVersion < 1 {

        }
    })

然后跑了

let config = Realm.Configuration(
    schemaVersion: 2,
    migrationBlock: {migration, oldSchemaVersion in
        if oldSchemaVersion < 1 {

        }
        
        if oldSchemaVersion < 2 {
            migration.deleteData(forType: "Bar")
        }
    })

结果奏效了。

这里的问题是，这是一种将两次迁移合并为一次迁移的方法吗？

【问题讨论】：

标签： realm-list realm-migration

【解决方案1】：

我找到了解决方案。只需将两个迁移一起应用即可。

let url = Realm.Configuration().fileURL!
let schemaVersion = try! schemaVersionAtURL(url)

if schemaVersion == 0 {
    autoreleasepool {
        let configuration = Realm.Configuration(
            // Set the new schema version. This must be greater than the previously used
            // version (if you've never set a schema version before, the version is 0).
            schemaVersion: 1,
            migrationBlock: { migration, oldSchemaVersion in
                // We haven’t migrated anything yet, so oldSchemaVersion == 0
                if (oldSchemaVersion < 1) {
                    
                }
        })
        _ = try! Realm(configuration: configuration)
    }
    
    autoreleasepool {
        let configuration = Realm.Configuration(
            // Set the new schema version. This must be greater than the previously used
            // version (if you've never set a schema version before, the version is 0).
            schemaVersion: 2,
            migrationBlock: { migration, oldSchemaVersion in
                // We haven’t migrated anything yet, so oldSchemaVersion == 0
                if (oldSchemaVersion < 1) {
                    
                }
                
                if (oldSchemaVersion < 2) {
                    migration.deleteData(forType: "Bar")
                }
        })
        _ = try! Realm(configuration: configuration)
    }
} else {
    let configuration = Realm.Configuration(schemaVersion: schemaVersion)
    _ = try! Realm(configuration: configuration)
}

autoreleasepool 部分是必需的。否则 schemaVersion 2 将不会被应用。

【讨论】：