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您可以使用 $lookup 为两个集合创建一个视图，然后对该视图运行查询。应用程序可以查询视图，而无需构建或维护复杂的管道。

例子

创建两个样本集合 inventory 和 orders：

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

创建连接视图

此命令使用 db.createView() 基于 orders 集合创建名为 sales 的新视图：

db.createView( "sales", "orders", [{$lookup:{from: "inventory",localField: "prodId",foreignField: "prodId",as: "inventoryDocs"}},{$project:{_id: 0,prodId: 1,orderId: 1,numPurchased: 1,price: "$inventoryDocs.price"}},{ $unwind: "$price" }
] )

在示例中：

• $lookup 阶段使用 orders 集合中的 prodId 字段来“连接” inventory 集合中具有匹配 prodId 字段的文档。

• 匹配的文档将作为数组添加到 inventoryDocs 字段中。

• $project 阶段选择可用字段的子集。

• $unwind 阶段会将 price 字段从数组转换为标量值。

sales 视图中的文档包括：

{ orderId: 201, prodId: 100, numPurchased: 20, price: 20 },
{ orderId: 202, prodId: 101, numPurchased: 10, price: 10 },
{ orderId: 203, prodId: 102, numPurchased: 5, price: 15 },
{ orderId: 204, prodId: 103, numPurchased: 15, price: 17 },
{ orderId: 205, prodId: 103, numPurchased: 20, price: 17 },
{ orderId: 206, prodId: 102, numPurchased: 1, price: 15 },
{ orderId: 207, prodId: 101, numPurchased: 5, price: 10 },
{ orderId: 208, prodId: 100, numPurchased: 10, price: 20 },
{ orderId: 209, prodId: 103, numPurchased: 30, price: 17 }

 查询视图

要查找每种产品的总销售量，请查询视图：

db.sales.aggregate( [{$group:{_id: "$prodId",amountSold: { $sum: { $multiply: [ "$price", "$numPurchased" ] } }}}
] )

输出见下：

[{ _id: 102, amountSold: 90 },{ _id: 101, amountSold: 150 },{ _id: 103, amountSold: 1105 },{ _id: 100, amountSold: 600 }
]

使用视图连接两个集合 — MongoDB 手册 v6.0