在 mongodb 中展开空数组 [英] Unwind empty array in mongodb

问题描述

这与 this question 非常相似，但是，当使用类似的方法时，我不断得到仅包含子文档的文档列表.

This is very similar to this question, however, when using a similar approach, I keep getting a list of only the documents that contain subdocuments.

我有一个简单的模式，其中的问题可以投票赞成和反对.我的架构(为简单起见，我删除了一些字段):

I have a simple schema with questions that can be up and down voted. My schema (I removed some fields for simplicity):

var mongoose = require('mongoose');

var voteSchema = new mongoose.Schema({
  value: Number,
});

// Document schema for polls
exports.PollSchema = new mongoose.Schema({
    question: { type: String, required: true },
    votes: [voteSchema],
});

我想要一个包含所有问题及其各自分数的列表.分数是 voteSchema 中值"字段的总和.

I want to have a list with all questions and their respective score. A score is the sum of the 'value' field in voteSchema.

我正在尝试使用 $cond 来确保将空投票文档替换为至少一个值为 0 的投票子文档.不幸的是，使用它时，我所有的 totalScore 字段都是 0.如果不使用它，我得到了正确的 totalScore，但仅适用于非空的.

I'm trying to use $cond to make sure the empty votes document is replaced with at least one vote subdocument with a value of 0. Unfortunately, when using it, all my totalScore fields are 0. If not using it, I get the correct totalScore, but only for the non empty ones.

Poll.aggregate([
    { $project: {
        _id: 1,
        question: 1,
        totalScore : 1,
        votes : { $cond : [ { $eq : ["$votes.length", 0]}, '$votes', [ { value : 0} ]] }
    }},
    { $unwind: "$votes" },
    { $group: {
        _id : "$_id",
        question: { $first: "$question" },
        totalScore : { $sum: "$votes.value" }
    }},
], function(error, polls) {
    console.dir(polls);
});

推荐答案

如果你的空votes是null(不存在)，尝试使用:

If your empty votes is null (not exists), try to use:

{ $ifNull : [ "$votes", [ { value : 0 } ] ] }  

代替:

{ $cond : [ { $eq : [ "$votes.length", 0 ] }, '$votes', [ { value : 0 } ] ] }  

如果你的空 votes 是 [](空数组):

If your empty votes is [] (empty array):

{ $cond : [ { $eq : [ "$votes", [] ] }, [ { value : 0 } ], '$votes' ] }  

还有一些关于 $cond 运算符:

And some about $cond operator:

它的语法:{ $cond: [ <boolean-expression>, <true-case>, <false-case>] }.

但是你用过，比如:{ $cond: [ <boolean-expression>, <false-case>, <true-case>] }.

如果 votes 不存在或为空数组(布尔条件为 true)，您应该使用 [ { value : 0} ] 代替它(将 [ { value : 0} ] 粘贴到 $cond 运算符数组的 2 项中).
此外，在这种情况下，length 属性不可访问.

If votes not exists or empty array (boolean condition is true) you should use [ { value : 0} ] instead of it (paste [ { value : 0} ] in 2 item of $cond operator array).
Also, length property is not accessable in this case.

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