blocks|key|755359|text|您需要使用$lookup来连接购买者id和用户id，对于所有匹配的集合，它将以嵌入文档的形式返回用户信息。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|755360|db.fruitz.aggregate(
++++[
++++++++{
++++++++++++$lookup+:+{
++++++++++++++++from+:+"userz",
++++++++++++++++localField+:+"buyer_id",
++++++++++++++++foreignField:+"_id",
++++++++++++++++as+:+"buyerInfo"
++++++++++++}
++++++++}
++++]
)|code-block|syntax|javascript|755361|水果收藏|755362|>+db.fruitz.find().pretty()
{+"_id"+:+2,+"title"+:+"apple",+"buyer_id"+:+1+}
{+"_id"+:+1,+"title"+:+"banana",+"buyer_id"+:+2+}|755363|users集合|755364|>+db.userz.find().pretty()
{+"_id"+:+1,+"name"+:+"abc",+"age"+:+20,+"gender"+:+"M"+}
>+|755365|$lookup聚合|755366|>+db.fruitz.aggregate(+[+{+$lookup+:+{+from+:+"userz",+localField+:+"buyer_id",+foreignField:+"_id",+as+:+"buyerInfo"+}+}+]+).pretty()|755367|输出|755368|{
++++"_id"+:+2,
++++"title"+:+"apple",
++++"buyer_id"+:+1,
++++"buyerInfo"+:+[
++++++++{
++++++++++++"_id"+:+1,
++++++++++++"name"+:+"abc",
++++++++++++"age"+:+20,
++++++++++++"gender"+:+"M"
++++++++}
++++]
}
{+"_id"+:+1,+"title"+:+"banana",+"buyer_id"+:+2,+"buyerInfo"+:+[+]+}
>+|755369|entityMap^0|5|7|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|12|8|@$9|13|A|14|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|15|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|16|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|17|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|18|8|@]|D|@]|E|$]]|$1|Q|3|R|5|H|7|19|8|@]|D|@]|E|$I|J]]|$1|S|3|T|5|6|7|1A|8|@]|D|@]|E|$]]|$1|U|3|V|5|H|7|1B|8|@]|D|@]|E|$I|J]]|$1|W|3|X|5|6|7|1C|8|@]|D|@]|E|$]]|$1|Y|3|Z|5|H|7|1D|8|@]|D|@]|E|$I|J]]|$1|10|3|-4|5|6|7|1E|8|@]|D|@]|E|$]]]|11|$]]

You need to use <code>$lookup</code> to join buyer id with user id, for all matching collections, it will return the user info as an embedded document

<pre><code>db.fruitz.aggregate(
 [
 {
 $lookup : {
 from : "userz",
 localField : "buyer_id",
 foreignField: "_id",
 as : "buyerInfo"
 }
 }
 ]
)
</code></pre>

fruits collection

<pre><code>&gt; db.fruitz.find().pretty()
{ "_id" : 2, "title" : "apple", "buyer_id" : 1 }
{ "_id" : 1, "title" : "banana", "buyer_id" : 2 }
</code></pre>

users collection

<pre><code>&gt; db.userz.find().pretty()
{ "_id" : 1, "name" : "abc", "age" : 20, "gender" : "M" }
&gt; 
</code></pre>

$lookup aggregate

<pre><code>&gt; db.fruitz.aggregate( [ { $lookup : { from : "userz", localField : "buyer_id", foreignField: "_id", as : "buyerInfo" } } ] ).pretty()
</code></pre>

output

<pre><code>{
 "_id" : 2,
 "title" : "apple",
 "buyer_id" : 1,
 "buyerInfo" : [
 {
 "_id" : 1,
 "name" : "abc",
 "age" : 20,
 "gender" : "M"
 }
 ]
}
{ "_id" : 1, "title" : "banana", "buyer_id" : 2, "buyerInfo" : [ ] }
&gt; 
</code></pre>

blocks|key|1074630|text|MongoDB提供了$lookup来连接来自两个集合的记录：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1074631|在这种情况下，您可以使用$lookup连接水果和用户|offset|length|1074632|db.fruits.aggregate([
+++{
+++++$lookup:
+++++++{
+++++++++from:+"fruits",
+++++++++localField:+"buyer",
+++++++++foreignField:+"_id",
+++++++++as:+"buyer_info"
+++++++}
++}
])|code-block|syntax|javascript|1074633|$lookup+hat在3.6中变得更强大了(如果你使用3.6)，它允许添加表达式和你从right集合连接的东西|1074634|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/#lookup-aggregation|1|https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/#specify-multiple-join-conditions-with-lookup^0|0|C|7|0|0|0|0|7|1|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|X|8|@]|9|@$D|Y|E|Z|1|10]]|A|$]]|$1|F|3|G|5|H|7|11|8|@]|9|@]|A|$I|J]]|$1|K|3|L|5|6|7|12|8|@]|9|@$D|13|E|14|1|15]]|A|$]]|$1|M|3|-4|5|6|7|16|8|@]|9|@]|A|$]]]|N|$O|$5|P|Q|R|A|$S|T]]|U|$5|P|Q|R|A|$S|V]]]]

MongoDB Provides $lookup to join records from two collections:

In your case you can join fruits and user using <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/#lookup-aggregation" rel="nofollow noreferrer">$lookup</a>:

<pre><code>db.fruits.aggregate([
 {
 $lookup:
 {
 from: "fruits",
 localField: "buyer",
 foreignField: "_id",
 as: "buyer_info"
 }
 }
])
</code></pre>

<a href="https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/#specify-multiple-join-conditions-with-lookup" rel="nofollow noreferrer">$lookup</a> hat got more powerful in 3.6(if you're using 3.6) which allows adding expressions and on the things you're joining from right collection

I have this two schema, fruits schema and user schema. I split them in 2 different collection, but a fruit schema can have a reference to a user.

<pre><code>const FruitSchema = new Schema({
 title: {
 type: String
 },
 buyer: {
 _id: {
 type: Schema.Types.ObjectId,
 default: null
 }
 }
})

const UserSchema = new Schema({
 name: {
 type: String
 },
 age: {
 type: Number
 },
 gender: {
 type: String
 }
})
</code></pre>

How would you query all fruits with the user info? What I can think of is find all fruits, get the buyer._id then find each user, map them back the fruit array, sounds so tedious and complicated. If it's mysql I just JOIN.

mongoose get relative data without query

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我有这两个模式，水果模式和用户模式。我将它们分成两个不同的集合，但是一个水果模式可以引用一个用户。const FruitSchema = new Schema({  title: {    type: String  },  buyer: {    _id: {      type: Schema.Types.Obje...

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