Parse.com 查询 10000 (10K) 个对象

Posted 2023-03-08

【中文标题】Parse.com 查询 10000 (10K) 个对象

【英文标题】：Parse.com query for 10000 (10K) objects

【发布时间】：2014-07-30 09:53:36

【问题描述】：

我有一个解析数据库，其中包含一个名为 MeetingObject 的类，其中填充了 6000 个对象（顺便说一下，它会增长...）。

作为解析查询限制 1000，我正在尝试使用跳过查询属性来获取它们。

以下代码给了我 2000 个对象：

NSMutableArray *allObjects = [NSMutableArray array];
NSUInteger limit = 1000;
__block NSUInteger skip = 0;
[query setLimit: limit];
[query setSkip: skip];
[query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) 
    if (!error) 
        [allObjects addObjectsFromArray:objects];
        NSLog(@"%lu", (unsigned long)allObjects.count );

        if (objects.count == limit) 
            // There might be more objects in the table. Update the skip value and execute the query again.
            skip += limit;
            [query setSkip: skip];
            [query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) 
                [allObjects addObjectsFromArray:objects];
                NSLog(@"%lu", (unsigned long)allObjects.count );
            ];
        

      else if (error || [error code] == kPFErrorConnectionFailed) 

        UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"Error!" message:NSLocalizedString(@"The Internet connection appears to be offline.",@"no internet") delegate:self cancelButtonTitle:nil otherButtonTitles:@"Ok", nil];
        self.navigationItem.rightBarButtonItem.enabled = YES;
        self.tableView.userInteractionEnabled = YES;
        [alertView show];
        return;
    
];

我的理解是，如果我想再获得 1000 个对象，我必须添加另一个嵌套查询，然后再添加一个用于下一个 1000 的对象，依此类推：

// finding the first 1000 objects
[query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) 
    if (!error) 
        [allObjects addObjectsFromArray:objects];
        NSLog(@"%lu", (unsigned long)allObjects.count );

        if (objects.count == limit) 
            // finding another 1000 objects
            skip += limit;
            [query setSkip: skip];
            [query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) 
                [allObjects addObjectsFromArray:objects];
                NSLog(@"%lu", (unsigned long)allObjects.count );

                if (objects.count == limit) 
                    // finding another 1000 objects
                    skip += limit;
                    [query setSkip: skip];
                    [query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) 
                        [allObjects addObjectsFromArray:objects];
                        NSLog(@"%lu", (unsigned long)allObjects.count );
                    ];
                

            ];

        

但是如果我不知道对象的确切数量怎么办？我尝试使用：

while (objects.count == limit) 
        // There might be more objects in the table. Update the skip value and execute the query again.
        skip += limit;
        [query setSkip: skip];
        [query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) 
            [allObjects addObjectsFromArray:objects];
            NSLog(@"%lu", (unsigned long)allObjects.count );
        ];
    

但我明白了

* 由于未捕获的异常“NSInternalInconsistencyException”而终止应用程序，原因：“此查询具有未完成的网络连接。你必须等到它完成。'

因为当然查询是在最后一个完成之前进行的......

【问题讨论】：

此链接回答您的问题：parse.com/questions/paging-through-more-than-10000-results

【参考方案1】：

虽然这是一个较晚的响应，但这可以帮助遇到此问题的其他人。 基本上，就像上面西蒙的回答一样，第 1 步是获取计数，然后创建一个组调度以循环遍历每个请求，直到所有数据都已下载。这是我使用的（稍微通用的）代码：

- (void)getLargeDataFromParseWithBlock:(void (^)(NSArray *, NSError *))block 
  int max = 1000;

  __block NSError *error;
  PFQuery *query = [PFQuery queryWithClassName:@"<your class>"];
  [query whereKey:@"<field name in class>" equalTo:@"xyz"];
  query.limit = max;
  // get count of ojects first
  [query countObjectsInBackgroundWithBlock:^(int count, NSError *_error) 
    if (!error) 
      // calculate how many requests are need based on count and max
      int APIrequests = (count+max-1)/max;
      // create a group dispatch
      dispatch_group_t downloadGroup = dispatch_group_create();
      for (int i=0; i<APIrequests; i++) 
        // tell dispatch a task is starting
        dispatch_group_enter(downloadGroup);
        PFQuery *dispatchQuery = [PFQuery queryWithClassName:@"<your class>"];
        [dispatchQuery whereKey:@"<field name in class>" equalTo:@"xyz"];
        dispatchQuery.limit = max;
        dispatchQuery.skip = i*max;
        [dispatchQuery findObjectsInBackgroundWithBlock:^(NSArray *arrayResponse, NSError *_error2) 
          if (!_error2) 
            NSLog(@"Successfully retrieved %lu.", (unsigned long)arrayResponse.count);

            // do something with arrayResponse like add to core data or sqlite

            // tell dispatch task has completed
            dispatch_group_leave(downloadGroup);
           else 
            NSLog(@"Error: %@ %@", _error2, [_error2 userInfo]);
            error = _error2;
            // tell dispatch task has completed - need to cover suuccess and fail scenarios of parse request
            dispatch_group_leave(downloadGroup);
          
        ];
      
      // called when no more tasks in dispatch
      dispatch_group_notify(downloadGroup, dispatch_get_main_queue(), ^
        if (block) 
          if (!error) 
            // array could contain combined responses - sourced from core data or sqlite for example
            block(@[], nil);
           else 
            block(nil, error);
          
        
      );
    else 
      block(nil, _error);
      NSLog(@"Count error: %@ %@", _error, [_error userInfo]);
    
  ];

【参考方案2】：

我要做的是先获取对象的数量，然后使用 skip 方法进行查询。

PFQuery *query = [PFQuery queryWithClassName:@"GameScore"];
[query whereKey:@"playername" equalTo:@"Sean Plott"];
[query countObjectsInBackgroundWithBlock:^(int count, NSError *error) 
  if (!error) 
     dispatchFindObjectsUsingLimit(count)
   else 
    // The request failed
  
];