假设我有 2 个收藏
// Post collection:
[
{
"_id": "somepost1",
"author": "firstuser",
"title": "First post"
},
{
"_id": "somepost2",
"author": "firstuser",
"title": "Second post"
},
{
"_id": "somepost3",
"author": "firstuser",
"title": "Third post"
}
]
// User collection:
[
{
"_id": "firstuser",
"nickname": "John",
"posts": {
"voted": []
}
},
{
"_id": "seconduser",
"nickname": "Bob",
"posts": {
"voted": [
{
"_id": "somepost1",
"vote": "1"
},
{
"_id": "somepost3",
"vote": "-1"
}
]
}
}
]
我需要得到这个结果:
[
{
"_id": "somepost1",
"author": {
"_id": "firstuser",
"nickname": "John"
},
"title": "First post",
"myvote": "1"
},
{
"_id": "somepost2",
"author": {
"_id": "firstuser",
"nickname": "John"
},
"title": "Second post",
"voted": "0"
},
{
"_id": "somepost3",
"author": {
"_id": "firstuser",
"nickname": "John"
},
"title": "Third post",
"myvote": "-1"
}
]
我如何使用聚合发出请求，该请求将使用元素的动态 _id 显示此输出？ 如果“posts.voted”中没有与当前帖子关联的元素，则在第二个 $match 中使用帖子的当前 _id 并将“myvote”设置为 0 时遇到问题。
这是我尝试过的:https://mongoplayground.net/p/v70ZUioVSpQ
db.post.aggregate([
{
$match: {
author: "firstuser"
}
},
{
$lookup: {
from: "user",
localField: "author",
foreignField: "_id",
as: "author"
}
},
{
$addFields: {
author: {
$arrayElemAt: [
"$author",
0
]
}
}
},
{
$lookup: {
from: "user",
localField: "_id",
foreignField: "posts.voted._id",
as: "Results"
}
},
{
$unwind: "$Results"
},
{
$unwind: "$Results.posts.voted"
},
{
$match: {
"Results.posts.voted._id": "ID OF CURRENT POST"
}
},
{
$project: {
_id: 1,
author: {
_id: 1,
nickname: 1
},
title: 1,
myvote: "$Results.posts.voted.vote"
}
}
])
最佳答案
来自$match docs :
The query syntax is identical to the read operation query syntax
查询语法不允许使用文档值。这就是你想要做的。
我们能做的就是使用$expr在 $match 阶段，这允许我们使用聚合运算符，从而也可以访问文档值。像这样:
{
$match: {
$expr: {
$eq: ['$Results.posts.voted._id', '$_id'],
}
},
},
关于mongodb - 如何在第二个$match中使用当前字段？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/71017475/