当你和数据库打交道的时候，你需要改变数据模型(Model)，但是因为Realm中的数据模型被定义为标准的Objective-C interfaces,要改变模型，就像改变其他Objective-C interface一样轻而易举。举个例子，假设有个数据模型Person:

@interface Person : RLMObject
@property NSString *firstName;
@property NSString *lastName;
@property int age;
@end

当我们想添加一个字段fullName属性而不是first和last names时，我们可以这样做:

@interface Person : RLMObject
@property NSString *fullName;
@property int age;
@end

接下来执行迁移:

// Inside your [AppDelegate didFinishLaunchingWithOptions:]

RLMRealmConfiguration *config = [RLMRealmConfiguration defaultConfiguration];
config.schemaVersion = 1;
config.migrationBlock = ^(RLMMigration *migration, uint64_t oldSchemaVersion) {
  //如果从没迁移过，oldSchemaVersion == 0
  if (oldSchemaVersion < 1) {
    // The enumerateObjects:block: method iterates
    // over every 'Person' object stored in the Realm file
    [migration enumerateObjects:Person.className
                          block:^(RLMObject *oldObject, RLMObject *newObject) {

      // 设置新增属性的值
      newObject[@"fullName"] = [NSString stringWithFormat:@"%@ %@",
                                         oldObject[@"firstName"],
                                         oldObject[@"lastName"]];
    }];
  }
};
[RLMRealmConfiguration setDefaultConfiguration:config];

在迁移的过程中重命名属性名称

// Inside your [AppDelegate didFinishLaunchingWithOptions:]

RLMRealmConfiguration *config = [RLMRealmConfiguration defaultConfiguration];
config.schemaVersion = 1;
config.migrationBlock = ^(RLMMigration *migration, uint64_t oldSchemaVersion) {
  // We haven’t migrated anything yet, so oldSchemaVersion == 0
  if (oldSchemaVersion < 1) {
    // The renaming operation should be done outside of calls to `enumerateObjects:`.
    [migration renamePropertyForClass:Person.className oldName:@"yearsSinceBirth" newName:@"age"];
  }
};
[RLMRealmConfiguration setDefaultConfiguration:config];

官方eg:

RLMRealmConfiguration *config = [RLMRealmConfiguration defaultConfiguration];
config.schemaVersion = 2;
config.migrationBlock = ^(RLMMigration *migration, uint64_t oldSchemaVersion)
{
    // enumerateObjects:block: 遍历了存储在 Realm 文件中的每一个“Person”对象
    [migration enumerateObjects:Person.className block:^(RLMObject *oldObject, RLMObject *newObject) {
        // 只有当 Realm 数据库的架构版本为 0 的时候，才添加 “fullName” 属性
        if (oldSchemaVersion < 1) {
            newObject[@"fullName"] = [NSString stringWithFormat:@"%@ %@", oldObject[@"firstName"], oldObject[@"lastName"]];
        }
        // 只有当 Realm 数据库的架构版本为 0 或者 1 的时候，才添加“email”属性
        if (oldSchemaVersion < 2) {
            newObject[@"email"] = @"";
        }
       // 替换属性名
       if (oldSchemaVersion < 3) { // 重命名操作应该在调用 `enumerateObjects:` 之外完成 
            [migration renamePropertyForClass:Person.className oldName:@"yearsSinceBirth" newName:@"age"]; }
    }];
};
[RLMRealmConfiguration setDefaultConfiguration:config];
// 现在我们已经成功更新了架构版本并且提供了迁移闭包，打开旧有的 Realm 数据库会自动执行此数据迁移，然后成功进行访问
[RLMRealm defaultRealm];