MongoDB:在graphlookup内部计算connectToField

问题描述

我有如下所示的mongo树结构:

I have mongo tree structure that looks like this:

{"parent": null, "path": "#a", "name": "a"}
{"parent": "a", "path": "#a#b", "name": "b"}
{"parent": "a", "path": "#a#c", "name": "c"}
{"parent": "b", "path": "#a#b#1", "name": "1"}
{"parent": "b", "path": "#a#b#2", "name": "2"}
{"parent": "c", "path": "#a#c#1", "name": "1"}
{"parent": "1", "path": "#a#c#1#x", "name": "x"}

可以表示如下:

#a
|_#a#b
|    |_#a#b#1
|    |_#a#b#2
|_#a#c
    |_#a#c#1
          |_#a#c#1#x

我的目标是建立一个仅获取指定节点下的叶子的请求.

My goal is to build a request that will get only the leaves under a specified node.

如果我已将父路径存储在字段parent内，而不是使用以下请求就可以做到的父标识符:

Had I stored the parent path inside the field parent instead of the parent identifier I would have been able to do it using the following request:

db.tree.aggregate([
        {$match:{"parent": {$regex:"^#a#c"}}}, 
        {$graphLookup:{
                       from:"tree", 
                       startWith:"$path", 
                       connectFromField:"path", 
                       connectToField:"parent", 
                       as:"dep"}}, 
        {$match:{dep:[]}}, 
        {$project:{"_id":0, path:1}}
])

在我之前的问题中已经回答过: Mongodb:仅获取树的叶子

as already answered in my previous question here:Mongodb: get only leaves of tree

问题是我没有.

因此，我必须以某种方式转换请求中的"connectToField"，以便它表示父代的路径而不是父代的ID.有人对如何执行此操作有想法吗?

So I have to somehow transform the 'connectToField' in my request so that it represents the path of my parent instead of the id of my parent.Does anybody have an idea on how to do this?

推荐答案

connectToField是名称，而不是表达式.你对此无能为力.

connectToField is a name, not an expression. You can't do anything with it.

您确实需要重新考虑架构.从父引用中使用的非唯一名称开始，它在许多方面都有缺陷.由于您依赖于路径字符串而不是，因此您需要一条路径来引用父项.

You really need to re-consider the schema. It is flawed in many ways starting from non-unique names used in parent references. Since you rely on the path string instead , you need a path to reference the parent.

以下答案是在运行时完成的，由于性能低下以及有关如何构建路径的一些假设，因此不建议用于操作查询.不过，它可以用作一次性请求.

The answer below does it runtime, and is hardly advisable for operational queries due to performance inefficiency and some assumptions of how the path is built. It can be used as a one-off request though.

基本上，您需要创建一个视图计算出的父路径:

Basically you need to create a view with calculated parent path:

db.createView("rootless_tree", "tree", [
    { $match: { parent: { $ne: null } } },
    { $addFields: {
        parent_path: { $let: {
            vars: { parents: { $split: [ "$path", "#" ] } },
            in: { $reduce: {
                input: { $slice: [ "$$parents", 1, { $subtract: [ { $size: "$$parents" }, 2 ] } ] },
                initialValue: "",
                in: { $concat: [ "$$value", "#", "$$this" ] }
            } }
        } }
    } }
]);

因此，您可以按照上一个问题的建议进行查找:

So then you can do your lookup as advised in your previous question:

db.tree.aggregate([
    { $graphLookup: {
        from: "rootless_tree", 
        startWith: "$path", 
        connectFromField: "path", 
        connectToField: "parent_path", 
        as:"dep"
    } },
    { $match: { dep: [] } },                       
])

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