添加数据

db.orders.insert([
   { "_id" : 1, "item" : "almonds", "price" : 12, "quantity" : 2 },
   { "_id" : 2, "item" : "pecans", "price" : 20, "quantity" : 1 },
   { "_id" : 3  }
])
db.inventory.insert([
   { "_id" : 1, "sku" : "almonds", "description": "product 1", "instock" : 120 },
   { "_id" : 2, "sku" : "bread", "description": "product 2", "instock" : 80 },
   { "_id" : 3, "sku" : "cashews", "description": "product 3", "instock" : 60 },
   { "_id" : 4, "sku" : "pecans", "description": "product 4", "instock" : 70 },
   { "_id" : 5, "sku": null, "description": "Incomplete" },
   { "_id" : 6 }
])

两表关联

db.orders.aggregate([
   {
     $lookup:
       {
         from: "inventory",
         localField: "item",
         foreignField: "sku",
         as: "inventory_docs"
       }
  }
])

三表关联

db.orders.aggregate([
   {
     $lookup:
       {
         from: "inventory",
         localField: "item",
         foreignField: "sku",
         as: "inventory_docs1"
       }
  },
  {
     $lookup:
       {
         from: "inventory",
         localField: "item",
         foreignField: "sku",
         as: "inventory_docs2"
       }
  }
])

多表关联

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