在 iOS 中使用 Parse Server 获取子对象和父对象的最佳方式

Question

我正在 Parse Server 上为 iOS 尝试 "Sample Blog App" 并且无法弄清楚获取另一个 class 的所有子对象的聪明方法是什么（连同父对象）。

"Sample Blog App"（在您创建新帐户时自动创建）包含 classes Comment 和 Post。评论 class 包含与 Post class 的关系，如下所示（来自仪表板），但没有相反方向的关系。

现在，我想获取 所有 post 以及与每个 post 相关的所有评论。下面的代码就是这样做的，但我假设一定有更聪明的方法......？如果你知道怎么做，请分享。提前致谢！

- (void)fetchPosts {

    NSString *commentsKey = @"comments";
    NSString *postKey = @"post";

    PFQuery *query = [PFQuery queryWithClassName:@"Comment"];
    [query includeKey:postKey];
    [query findObjectsInBackgroundWithBlock:^(NSArray * _Nullable objects, NSError * _Nullable error) {

        if (error == nil) {

            NSMutableArray *array = [[NSMutableArray alloc]init];

            for (PFObject *comment in objects) {

                PFObject *post = [comment objectForKey:postKey];
                NSDictionary *existingPostDict = [[array filteredArrayUsingPredicate:[NSPredicate predicateWithFormat:@"%K = %@", @"post.objectId", post.objectId]] firstObject];

                if (existingPostDict) {
                    // update comments
                    NSArray *comments = [[existingPostDict objectForKey:commentsKey] arrayByAddingObject:comment];

                    // create new dictionary and replace the old one
                    NSDictionary *newPostDict = [[NSDictionary alloc]initWithObjectsAndKeys:[existingPostDict objectForKey:postKey], postKey, comments, commentsKey, nil];
                    [array replaceObjectAtIndex:[array indexOfObject:existingPostDict] withObject:newPostDict];
                }
                else {
                    // first post, create a new dict
                    NSDictionary *newPostDict = [[NSDictionary alloc]initWithObjectsAndKeys:post, postKey, @[comment], commentsKey, nil];
                    [array addObject:newPostDict];
                }
            }
            self.posts = array; // assuming: @property (nonatomic, strong) NSArray *posts; 
        }
        else {
            NSLog(@"Error fetching posts: %@", error.localizedDescription);
        }
    }];
}

Answer 1

不应在查询中使用 include，而应使用 whereKey:equals: 并将 post 对象作为第二个参数传递。这将过滤并 return 仅包含将 post 作为其值 post

的评论对象

Answer 2

我在您的查询中看到的一个问题是，这可能不会获取数据库中的每个 post。如果 post 有 0 个评论，none 个 Comment 对象将引用它，因此您不会收到它。

因此，您实际上应该对 "Post" 进行查询，并在完成后对 "Comment" 进行查询。这样你就不会错过任何评论为 0 的 post。执行此操作时，您不需要在 Comment 查询中包含 "post" 键。这有很多好处。

首先，每个包含也是对该对象的另一个查询。因此，每个新的 Comment 对象都会在后端创建另一个查询。你会自动摆脱它。

其次，对于具有多个评论的 "Post"，您将多次查询相同的 post，并且相同的 post 将被返回多次，这会消耗不必要的带宽。

分别获取帖子和评论后，只需将它们合并即可。

除此之外，我会像这样进行组合，我觉得这样更具可读性，但这只是个人喜好。

- (void)fetchPosts {

NSString *commentsKey = @"comments";
NSString *postKey = @"post";

PFQuery *query = [PFQuery queryWithClassName:@"Comment"];
[query includeKey:postKey];
[query findObjectsInBackgroundWithBlock:^(NSArray * _Nullable objects, NSError * _Nullable error) {

    if (error == nil) {

        NSMutableArray *array = [[NSMutableArray alloc]init];
        NSMutableDictionary *d = [NSMutableDictionary dictionary];
        for (PFObject *comment in objects) {

            PFObject *post = [comment objectForKey:postKey];
            if (d[post.objectId]) {
                [d[post.objectId][commentsKey] addObject:comment];
            }
            else{
                d[post.objectId] = [NSMutableDictionary dictionary];
                d[post.objectId][postKey]=post;
                d[post.objectId][commentsKey] = [NSMutableArray arrayWithObject:comment];
            }

        }
        for (NSString *key in [d allKeys]) {
            [array addObject:d[key]];
        }

        self.posts = array; // assuming: @property (nonatomic, strong) NSArray *posts;
    }
    else {
        NSLog(@"Error fetching posts: %@", error.localizedDescription);
    }
}];
}

Answer 3

我就是这样做的，使用 findObjectsInBackground 和 continueWithSuccessBlock: 方法（为了更好地处理错误，可以选择 continueWithBlock:）：

- (void)fetchPosts {
    /**
     create "post" and "comment" queries and use a BFTask-method from 
     Bolts.framework to chain downloading tasks together (bundled with Parse SDK)
     */
    NSMutableArray *posts = [NSMutableArray new];
    PFQuery *postQuery = [PFQuery queryWithClassName:@"Post"];
    [[[postQuery findObjectsInBackground] continueWithSuccessBlock:^id(BFTask * task) {

        [posts addObjectsFromArray:task.result];
        PFQuery *commentsQuery = [PFQuery queryWithClassName:@"Comment"];
        return [commentsQuery findObjectsInBackground];
    }] continueWithSuccessBlock:^id(BFTask * task) {
        /**
         loop through posts and filter out comments with the same objectId in post,
         then create a dictionary with post and related comments. done! :)
         */
        NSMutableArray *postsAndComments = [NSMutableArray new];
        for (PFObject *post in posts) {
            NSArray *comments = [task.result filteredArrayUsingPredicate:[NSPredicate predicateWithFormat:@"%K == %@", @"post.objectId", post.objectId]];
            [postsAndComments addObject:@{@"post":post, @"comments":comments}];
        }
        /**
         note: BFTask-blocks not running in main thread!
        */
        dispatch_async(dispatch_get_main_queue(), ^{
            self.posts = postsAndComments;  // assuming: @property (nonatomic, strong) NSArray *posts;
        });
        return nil;
    }];
}

在 iOS 中使用 Parse Server 获取子对象和父对象的最佳方式

Best way to fetch child and parent objects with Parse Server in iOS

objective-c

nspredicate

ios

parse-platform

back4app