Node.js - MongoDB Join_Node.js 菜鸟教程

MongoDB 是一个 NoSQL 数据库，它不支持在 MySQL 等关系数据库中发现的 JOIN 操作。但是，可以通过调用 Collection 对象的 aggregate（）方法和 $lookup 阶段来实现类似的功能。

$aggregate（）函数

此函数执行到同一数据库中的另一个集合的左外联接，以筛选“joined”集合中的文档进行处理。

$lookup：对同一数据库中的集合执行左外联接，以筛选“joined”集合中的文档进行处理。$lookup阶段将新的数组字段添加到每个输入文档。新的数组字段包含“joined”集合中的匹配文档。

为了在输入文档中的字段与“联接”集合的文档中的字段之间执行相等匹配，$lookup阶段具有以下语法 -

{
	 	$lookup:
	 	 	 {
	 	 	 	 	from: <collection to join>,
	 	 	 	 	localField: <field from the input documents>,
	 	 	 	 	foreignField: <field from the documents of the "from" collection>,
	 	 	 	 	as: <output array field>
	 	 	 }
}

$lookup阶段的参数如下 -

参数	描述
from	指定要执行联接的同一数据库中的集合。 from 是可选的，您可以在 $lookup 阶段中使用 $documents 阶段。有关示例，请参阅在 $lookup 阶段中使用$documents阶段。
localField	指定从文档输入到$lookup阶段的字段。$lookup from collection 的文档中，对 localField 和 foreignField 执行相等匹配。
foreignField	指定 from 集合中文档的字段。
As	指定要添加到输入文档的新数组字段的名称。新的数组字段包含 from 集合中的匹配文档。

$lookup操作对应于以下SQL查询 -

SELECT *, <output array field>
FROM collection
WHERE <output array field> IN (
	 	SELECT *
	 	FROM <collection to join>
	 	WHERE <foreignField> = <collection.localField>
);

例子

要演示 MongoDB 中的 JOIN 操作，请创建两个集合 - inventory 和 orders。

库存集合

( [
	 	{ prodId: 100, price: 20, quantity: 125 },
	 	{ prodId: 101, price: 10, quantity: 234 },
	 	{ prodId: 102, price: 15, quantity: 432 },
	 	{ prodId: 103, price: 17, quantity: 320 }
] )

订单集合

( [
	 	{ orderId: 201, custid: 301, prodId: 100, numPurchased: 20 },
	 	{ orderId: 202, custid: 302, prodId: 101, numPurchased: 10 },
	 	{ orderId: 203, custid: 303, prodId: 102, numPurchased: 5 },
	 	{ orderId: 204, custid: 303, prodId: 103, numPurchased: 15 },
	 	{ orderId: 205, custid: 303, prodId: 103, numPurchased: 20 },
	 	{ orderId: 206, custid: 302, prodId: 102, numPurchased: 1 },
	 	{ orderId: 207, custid: 302, prodId: 101, numPurchased: 5 },
	 	{ orderId: 208, custid: 301, prodId: 100, numPurchased: 10 },
	 	{ orderId: 209, custid: 303, prodId: 103, numPurchased: 30 }
] )

以下代码在 Collection 对象和 $lookup 阶段上调用 aggregate（）方法。

const {MongoClient} = require('mongodb');

async function main(){

	 	const uri = "mongodb://localhost:27017/";
	 	const client = new MongoClient(uri);

	 	try {
	 	 	 await client.connect();
	 	 	 await joindocs(client, "mydb", "orders", "inventory");
	 	} finally {
	 	 	 await client.close();
	 	}
}

main().catch(console.error);

async function joindocs(client, dbname, col1, col2){
	 	const result = await client.db(dbname).collection('orders').aggregate([
	 	{ $lookup:
	 	 	 {
	 	 	 	 	from: 'inventory',
	 	 	 	 	localField: 'prodId',
	 	 	 	 	foreignField: 'prodId',
	 	 	 	 	as: 'orderdetails'
	 	 	 }
	 	}
	 	]).toArray();
	 	result.forEach(element => {
	 	 	 console.log(JSON.stringify(element));
	 	});
}

$lookup阶段允许您指定要与当前集合联接的集合，以及应匹配的字段。

输出

{"_id":"658c4b14943e7a1349678bf3","orderId":201,"custid":301,"prodId":100,"numPurchased":20,"orderdetails":[{"_id":"658c4aff943e7a1349678bef","prodId":100,"price":20,"quantity":125}]}
{"_id":"658c4b14943e7a1349678bf4","orderId":202,"custid":302,"prodId":101,"numPurchased":10,"orderdetails":[{"_id":"658c4aff943e7a1349678bf0","prodId":101,"price":10,"quantity":234}]}
{"_id":"658c4b14943e7a1349678bf5","orderId":203,"custid":303,"prodId":102,"numPurchased":5,"orderdetails":[{"_id":"658c4aff943e7a1349678bf1","prodId":102,"price":15,"quantity":432}]}
{"_id":"658c4b14943e7a1349678bf6","orderId":204,"custid":303,"prodId":103,"numPurchased":15,"orderdetails":[{"_id":"658c4aff943e7a1349678bf2","prodId":103,"price":17,"quantity":320}]}
{"_id":"658c4b14943e7a1349678bf7","orderId":205,"custid":303,"prodId":103,"numPurchased":20,"orderdetails":[{"_id":"658c4aff943e7a1349678bf2","prodId":103,"price":17,"quantity":320}]}
{"_id":"658c4b14943e7a1349678bf8","orderId":206,"custid":302,"prodId":102,"numPurchased":1,"orderdetails":[{"_id":"658c4aff943e7a1349678bf1","prodId":102,"price":15,"quantity":432}]}
{"_id":"658c4b14943e7a1349678bf9","orderId":207,"custid":302,"prodId":101,"numPurchased":5,"orderdetails":[{"_id":"658c4aff943e7a1349678bf0","prodId":101,"price":10,"quantity":234}]}
{"_id":"658c4b14943e7a1349678bfa","orderId":208,"custid":301,"prodId":100,"numPurchased":10,"orderdetails":[{"_id":"658c4aff943e7a1349678bef","prodId":100,"price":20,"quantity":125}]}
{"_id":"658c4b14943e7a1349678bfb","orderId":209,"custid":303,"prodId":103,"numPurchased":30,"orderdetails":[{"_id":"658c4aff943e7a1349678bf2","prodId":103,"price":17,"quantity":320}]}

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$aggregate（） 函数

例子

库存集合

$aggregate（）函数