Mongodb $lookup 用法简单介绍
<p><strong>$lookup</strong> 是 Mongodb 3.2版本 新增的聚合框架中的一种查询方式； 主要用来实现多表链接查询； 相当关系型数据库中多表链接查询。</p><p>一、基本语法及主要功能</p>
<p><strong>1. 主要功能 </strong>是将每个输入待处理的文档，经过$lookup 阶段的处理，输出的新文档中会包含一个新生成的数组列（户名可根据需要命名新key的名字 ）。数组列存放的数据 是 来自 被Join 集合的适配文档，如果没有，集合为空（即 为[ ]）</p>
<p><strong>2.主要语法：</strong></p>
<div class="cnblogs_Highlighter">
<pre class="brush:csharp;gutter:true;">db.collection.aggregate([{
$lookup: {
from: "<collection to join>",
localField: "<field from the input documents>",
foreignField: "<field from the documents of the from collection>",
as: "<output array field>"
}
})
</pre>
</div>
<p> </p>
<p><strong>3.语法的解释说明</strong></p>
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<tbody>
<tr>
<td><strong>语法值</strong></td>
<td><strong>解释说明</strong></td>
</tr>
<tr>
<td>
<pre>from</pre>
</td>
<td>同一个数据库下等待被Join的集合。</td>
</tr>
<tr>
<td>
<pre>localField</pre>
</td>
<td>
<p>源集合中的match值，如果输入的集合中，某文档没有 localField</p>
<p>这个Key（Field），在处理的过程中，会默认为此文档含</p>
<p>有 localField：null的键值对。</p>
</td>
</tr>
<tr>
<td>
<pre>foreignField</pre>
</td>
<td>待Join的集合的match值，如果待Join的集合中，文档没有foreignField<br>值，在处理的过程中，会默认为此文档含有 foreignField：null的键值对。</td>
</tr>
<tr>
<td>
<pre>as</pre>
</td>
<td>为输出文档的新增值命名。如果输入的集合中已存在该值，则会覆盖掉，</td>
</tr>
</tbody>
</table>
<p> </p>
<p>二、两张表关联查询</p>
<p>1.测试数据准备</p>
<div class="cnblogs_code">
<pre>db.Customer.insert({customerCode:1,name:"customer1",phone:"13112345678",address:"test1"<span>})
db.Customer.insert({customerCode:2,name:"customer2",phone:"13112345679",address:"test2"<span>})
db.Order.insert({orderId:1,orderCode:"order001",customerCode:1,price:200<span>})
db.Order.insert({orderId:2,orderCode:"order002",customerCode:2,price:400})</span></span></span></pre>
</div>
<p>2. $lookup 查询两张表</p>
<div class="cnblogs_code">
<pre><span>db.Customer.aggregate(
[{ $lookup: {
from: "Order"<span>,
localField: "customerId"<span>,
foreignField: "customerId"<span>,
as: "customerOder"<span>
}
} <span>
])</span></span></span></span></span></span></pre>
</div>
<p>效果如下：</p>
<p><img src="https://img2020.cnblogs.com/blog/2440562/202107/2440562-20210726134854180-294097100.png"></p>
<p> </p>
<p> </p>
<p> $lookup 中的as:"customerOrder" 聚合为 一个数组类型字段</p>
<p> </p>
<p>三、三张表关联查询</p>
<p>1.准备数据</p>
<div class="cnblogs_code">
<pre>db.Customer.insert({customerCode:1,name:"customer1",phone:"13112345678",address:"test1"<span>})
db.Customer.insert({customerCode:2,name:"customer2",phone:"13112345679",address:"test2"<span>})
db.Order.insert({orderId:1,orderCode:"order001",customerCode:1,price:200<span>})
db.Order.insert({orderId:2,orderCode:"order002",customerCode:2,price:400<span>})
db.orderItem.insert({itemId:1,productName:"apples",qutity:2,orderId:1<span>})
db.orderItem.insert({itemId:2,productName:"oranges",qutity:2,orderId:1<span>})
db.orderItem.insert({itemId:3,productName:"mangoes",qutity:2,orderId:1<span>})
db.orderItem.insert({itemId:4,productName:"apples",qutity:2,orderId:2<span>})
db.orderItem.insert({itemId:5,productName:"oranges",qutity:2,orderId:2<span>})
db.orderItem.insert({itemId:6,productName:"mangoes",qutity:2,orderId:2})</span></span></span></span></span></span></span></span></span></pre>
</div>
<p>2.$lookup 查询三张表</p>
<div class="cnblogs_code">
<pre><span> db.Order.aggregate([{
$lookup: {
from: "Customer"<span>,
localField: "customerCode"<span>,
foreignField: "customerCode"<span>,
as: "curstomer"<span>
}
},
<strong> {
$lookup: {
</strong><strong>from: "orderItem"<span>,
localField: "orderId"<span>,
foreignField: "orderId"<span>,
as: "orderItem"</span></span></span></strong><span><strong>
}
}</strong>
}]
</span></span></span></span></span></span></pre>
</div>
<p>效果如下：</p>
<p><img src="https://img2020.cnblogs.com/blog/2440562/202107/2440562-20210726135633196-1224692953.png"></p>
<p> </p>
<p> $lookup 中的as:"customerOrder" 聚合为 一个数组类型字段</p>
<p>$lookup 中的as:"orderItem" 聚合为 一个数组类型字段</p>
<p> </p>
<p>由此可见，关联多少个人表，就可以使用多少个$lookup来做关联；</p>
<p>至此 $lookup 简单用法介绍完毕！</p>
<h2> </h2>
</div>
<p>1.测试数据准备</p><br><br>
来源：https://www.cnblogs.com/howardxu/p/15061192.html
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