app中使用了CoreData,并且在下一個版本中有實體變動，比如實體新增字段、修改字段等改動，那么app在覆蓋安裝時就要進行數據庫遷移，否則app就會crash

//1、創建模型對象
    //獲取模型路徑
    NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"Model" withExtension:@"momd"];
    //根據模型文件創建模型對象
    NSManagedObjectModel *model = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];
    
    
    //2、創建持久化助理
    //利用模型對象創建助理對象
    NSPersistentStoreCoordinator *store = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:model];
    
    //數據庫的名稱和路徑
    NSString *docStr = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) lastObject];
    NSString *sqlPath = [docStr stringByAppendingPathComponent:@"mySqlite.sqlite"];
    NSLog(@"path = %@", sqlPath);
    NSURL *sqlUrl = [NSURL fileURLWithPath:sqlPath];
    
    NSDictionary *options = @{NSMigratePersistentStoresAutomaticallyOption:@(YES),NSInferMappingModelAutomaticallyOption:@(YES)};
    //設置數據庫相關信息
    [store addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:sqlUrl options:options error:nil];
    
    //3、創建上下文
    NSManagedObjectContext *context = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
    
    //關聯持久化助理
    [context setPersistentStoreCoordinator:store];
    
    _context = context;

NSDictionary *options = @{NSMigratePersistentStoresAutomaticallyOption:@(YES),NSInferMappingModelAutomaticallyOption:@(YES)};
    //設置數據庫相關信息
[store addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:sqlUrl options:options error:nil];