MongoDB嵌套集 [英] MongoDB nested sets

问题描述

在MongoDB中存储嵌套集(如注释树)的最佳实践是什么?

What're the best practices to store nested sets (like trees of comments) in MongoDB?

我的意思是，每个评论都可以有一个父评论和一个子评论(答案).

I mean, every comment can have a parent comment and children-comments (answers).

像这样存储它们:

{
   title: "Hello",
   body: "Please comment me!",
   comments: [
        {
            author: "Peter",
            text: "Hi there",
            answers: [
                  {
                      author: "Peter",
                      text: "Hi there",
                      answers: [
                                 { author: "Ivan", text: "Hi there" },
                                 { author: "Nicholas", text: "Hi there" }
                      ]
                  },
                  { author: "Ivan", text: "Hi there" },
                  { author: "Nicholas", text: "Hi there" },
            ]
        },
        { author: "Ivan", text: "Hi there" },
        { author: "Nicholas", text: "Hi there" },
   ]
}

并不酷，因为例如，我们不能在没有地图/缩小的情况下索取彼得发表评论的所有帖子".

is not cool, because we can't, for example, ask for "all post which are commented by Peter" without map/reduce.

推荐答案

我认为没有完美的解决方案-取决于哪些操作对您的应用更为重要.我认为，例如Silicon Alley Insider会存储嵌套在MongoDB中的注释.这确实使您提到的查询更加困难.

I think there is no perfect solution - depends on what operations are more important for your app. I believe Silicon Alley Insider stores comments nested with MongoDB for example. That does make the query you mention harder.

一个选项存储在帖子的顶层，其中包含数组中所有评论者的列表.可以将其视为非规范化数据.这样一来，您可以轻松找到涉及某个评论者的所有帖子.然后向下钻取，您可以使用map/reduce或db.eval()来获取嵌套的帖子信息.

One option is store at top-level in the post a list of all commenters in an array. Think of that as denormalized data. Then one can easily find all posts which involve a certain commenter. Then to drill down, you use map/reduce or db.eval() to get the nested post info within.

另一个注意事项-如果要处理单个文档，则db.eval()的重量可能比map/reduce的重量轻. $ where也是一个选项，但可能会很慢，因此我喜欢上面提到的其他评论者列表"-也不容易为该数组建立​​索引(请参见文档中的"Multikey").

One other note - if you are dealing with a single document, db.eval() is probably lighter-weight than map/reduce. $where is also an option but can be slow so I like the additional 'list of commenters' mentioned above - not it is also easy to index that array too (see 'Multikey' in the docs).

另请参阅: http://groups.google.com/group/mongodb-user/browse_thread/thread/df8250573c91f75a/e880d9c57e343b52?lnk=gst&q=trees#e880d9c57e343b52

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