首先让我向您介绍我正在使用的 2 个集合:
集合 1:用户
> db.users.find().pretty()
{
"_id" : ObjectId("5ee4e727d04e4b4ac1ef115b"),
"name" : "Ashutosh Tiwari",
"age" : 21,
"email" : "<a href="https://stackoverflow.com/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="a9c8dac1dcddc6dac1e9cec4c8c0c587cac6c4" rel="noreferrer noopener nofollow">[email protected]</a>"
}
{
"_id" : ObjectId("5ee4e727d04e4b4ac1ef115c"),
"name" : "Maximilian",
"age" : 32,
"email" : "<a href="https://stackoverflow.com/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="bcd1ddc4fcc5ddd4d3d392dfd3d1" rel="noreferrer noopener nofollow">[email protected]</a>"
}
集合 2:帖子
> db.posts.find().pretty()
{
"_id" : ObjectId("5ee51b7ed9f661cad505fcc6"),
"title" : "First One",
"text" : "Hey this is the first Author",
"author" : ObjectId("5ee4e727d04e4b4ac1ef115c"),
"comments" : [
{
"user" : ObjectId("5ee4e727d04e4b4ac1ef115b"),
"comment" : "This is my comment"
}
]
}
{
"_id" : ObjectId("5ee5353cd9f661cad505fcc8"),
"title" : "First One",
"author" : ObjectId("5ee4e727d04e4b4ac1ef115c"),
"comments" : [
{
"user" : ObjectId("5ee4e727d04e4b4ac1ef115b"),
"comment" : "This is my comment"
}
]
}
我想让第二个集合(帖子)中的评论数组内的用户被撰写该评论的用户替换。
我已经尝试了下面的查询，但它正在替换评论部分!
> db.posts.aggregate([
{ $lookup:
{from: "users",
localField:"comments.user",
foreignField:"_id",
as:"comments.user"
}
} ]).pretty()
{
"_id" : ObjectId("5ee51b7ed9f661cad505fcc6"),
"title" : "First One",
"text" : "Hey this is the first Author",
"author" : ObjectId("5ee4e727d04e4b4ac1ef115c"),
"comments" : {
"user" : [
{
"_id" : ObjectId("5ee4e727d04e4b4ac1ef115b"),
"name" : "Ashutosh Tiwari",
"age" : 21,
"email" : "<a href="https://stackoverflow.com/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="6d0c1e051819021e052d0a000c0401430e0200" rel="noreferrer noopener nofollow">[email protected]</a>"
}
]
}
}
{
"_id" : ObjectId("5ee5353cd9f661cad505fcc8"),
"title" : "First One",
"author" : ObjectId("5ee4e727d04e4b4ac1ef115c"),
"comments" : {
"user" : [
{
"_id" : ObjectId("5ee4e727d04e4b4ac1ef115b"),
"name" : "Ashutosh Tiwari",
"age" : 21,
"email" : "<a href="https://stackoverflow.com/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="2445574c51504b574c644349454d480a474b49" rel="noreferrer noopener nofollow">[email protected]</a>"
}
]
}
}
因此，在这里，整个评论部分现已被替换，而我希望在 comments.user 部分中获得详细信息，以便我可以看到评论和发布该评论的用户。
最佳答案
您需要先展开评论数组
您的查询可能如下所示
db.posts.aggregate([
{
$unwind: "$comments" // unwind the comments array to get a stream of documents, each document has only one comment
},
{
$lookup: {
from: "users",
localField: "comments.user",
foreignField: "_id",
as: "comments.user"
}
},
{
$unwind: "$comments.user" // we know there is only one user inside a single comment, so we can unwind this user array to be an object too (as the lookup returns an array)
},
{
$group: { // then do a group by the document _id to get unique documents with comments array instead of the same document duplicated with different comments
_id: "$_id",
author: {
$first: "$author"
},
text: {
$first: "$text"
},
title: {
$first: "$title"
},
comments: {
$push: "$comments"
}
}
}
])
你可以 test it here
希望对你有帮助
关于mongodb - 为什么 $lookup 中的 "as"会替换整个集合？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/62406476/