iOS核心数据：获取比保存更多的对象

Question

我是Core Data的新手，我正在尝试将其用于保存用户。但是，当我第一次创建用户时，它会插入用户。这似乎没问题。它只被调用一次。

-(void)insertUser: (User *) user{
    [self userToModel:user];
    NSError *error = nil;
    if(![context save:&error]){
        NSLog(@"Unable to save managed object context");
    }
}

-(UserModel *)userToModel:(User *) user{
    UserModel *usermodel = [NSEntityDescription
                            insertNewObjectForEntityForName:@"UserModel"
                            inManagedObjectContext:context];

    [usermodel setValue:user.username forKey:@"username"];
    [usermodel setValue:user.password forKey:@"password"];
    [usermodel setValue:user.sessionid forKey:@"sessionid"];
    [usermodel setValue:[NSNumber numberWithBool:TRUE] forKey:@"selected"];
    [usermodel setValue:[NSNumber numberWithBool:user.beta] forKey:@"beta"];

    return usermodel;
}

接下来，我正在获取用户并返回3个用户：2个完全相似的用户和一个零模型。

-(NSMutableArray *)getUsers{
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"UserModel"
                                              inManagedObjectContext:context];
    NSError *error;
    [fetchRequest setEntity:entity];
    NSArray *usermodels= [context executeFetchRequest:fetchRequest error:&error];
    NSMutableArray *users = [[NSMutableArray alloc] init];
    for(UserModel *userModel in usermodels){
        User *user = [self userModelToUser:userModel];
        [users addObject:user];
    }
    return users;
}

UserModelToUser：

-(User *)userModelToUser:(UserModel *)usermodel{
    User *user = [[User alloc]initWithUsername:usermodel.username
                                      Password:usermodel.password
                                     CSRFToken:usermodel.csrftoken
                                     sessionid:usermodel.sessionid
                                          Beta:[usermodel.beta boolValue]
                                      Selected:[usermodel.selected boolValue]];
    return user;
}

SelectUser

-(void)selectUser: (User *)userToSelect OldUser:(User *)oldSelectedUser{
    UserModel *oldSelectedUserModel = [self userToModel:oldSelectedUser];
    UserModel *newSelectedUsermodel = [self userToModel:userToSelect];
    [oldSelectedUserModel setValue:[NSNumber numberWithBool:FALSE] forKey:@"selected"];
    [newSelectedUsermodel setValue:[NSNumber numberWithBool:TRUE] forKey:@"selected"];

    NSError *saveError;
    if (![context save:&saveError]) {
        NSLog(@"Changing users failed %@", saveError);
    }
}

我已经尝试重新安装该应用，以确保没有旧用户仍然保存或任何东西。任何人都可以帮助我吗？