参考：官方文档

    1. db.orders.insert([
    2.    { "_id" : 1, "item" : "almonds", "price" : 12, "quantity" : 2 },
    3.    { "_id" : 2, "item" : "pecans", "price" : 20, "quantity" : 1 },
    4.    { "_id" : 3  }
    5. ])
    7. db.inventory.insert([
    8.    { "_id" : 1, "sku" : "almonds", "description": "product 1", "instock" : 120 },
    9.    { "_id" : 2, "sku" : "bread", "description": "product 2", "instock" : 80 },
    10.    { "_id" : 3, "sku" : "cashews", "description": "product 3", "instock" : 60 },
    11.    { "_id" : 4, "sku" : "pecans", "description": "product 4", "instock" : 70 },
    12.    { "_id" : 5, "sku": null, "description": "Incomplete" },
    13.    { "_id" : 6 }
    14. ])
    1. '''
    2. SELECT *, inventory_docs
    3. FROM orders
    4. WHERE inventory_docs IN (SELECT *
    5. FROM inventory
    6. WHERE sku= orders.item);
    7. '''
    9. db.orders.aggregate([
    10.    {
    11.      $lookup:
    12.        {
    13.          from: "inventory",
    14.          localField: "item",
    15.          foreignField: "sku",
    16.          as: "inventory_docs"
    17.        }
    18.   }
    19. ])
    1. {
    2.    "_id" : 1,
    3.    "item" : "almonds",
    4.    "price" : 12,
    5.    "quantity" : 2,
    6.    "inventory_docs" : [
    7.       { "_id" : 1, "sku" : "almonds", "description" : "product 1", "instock" : 120 }
    8.    ]
    9. }
    10. {
    11.    "_id" : 2,
    12.    "item" : "pecans",
    13.    "price" : 20,
    14.    "quantity" : 1,
    15.    "inventory_docs" : [
    16.       { "_id" : 4, "sku" : "pecans", "description" : "product 4", "instock" : 70 }
    17.    ]
    18. }
    19. {
    20.    "_id" : 3,
    21.    "inventory_docs" : [
    22.       { "_id" : 5, "sku" : null, "description" : "Incomplete" },
    23.       { "_id" : 6 }
    24.    ]
    25. }