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
