Question

我在汇总查询中使用了$lookup。但正如我所看到的那样，它是LEFT OUTER JOIN。

我想用$lookup获取完全匹配的文档（INNER JOIN）。

有没有办法完成它？

这是我的inventory集合：

/* 1 */
{
    "_id" : 1,
    "sku" : "abc",
    "description" : "product 1",
    "instock" : 120
}

/* 2 */
{
    "_id" : 2,
    "sku" : "def",
    "description" : "product 2",
    "instock" : 80
}

/* 3 */
{
    "_id" : 3,
    "sku" : "ijk",
    "description" : "product 3",
    "instock" : 60
}

/* 4 */
{
    "_id" : 4,
    "sku" : "jkl",
    "description" : "product 4",
    "instock" : 70
}

/* 5 */
{
    "_id" : 5,
    "sku" : null,
    "description" : "Incomplete"
}

这是我的orders集合

/* 1 */
{
    "_id" : 1,
    "item" : "abc",
    "price" : 12,
    "quantity" : 2
}

/* 2 */
{
    "_id" : 2,
    "item" : "jkl",
    "price" : 20,
    "quantity" : 1
}

/* 3 */
{
    "_id" : 10,
    "item" : "jklw",
    "price" : 20,
    "quantity" : 1
}

这是查询

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

在此查询中，我将inventory's个文档与orders个文档匹配

预期结果

/* 1 */
{
    "_id" : 1,
    "sku" : "abc",
    "description" : "product 1",
    "instock" : 120,
    "inventory_docs" : [ 
        {
            "_id" : 1,
            "item" : "abc",
            "price" : 12,
            "quantity" : 2
        }
    ]
}

/* 2 */
{
    "_id" : 4,
    "sku" : "jkl",
    "description" : "product 4",
    "instock" : 70,
    "inventory_docs" : [ 
        {
            "_id" : 2,
            "item" : "jkl",
            "price" : 20,
            "quantity" : 1
        }
    ]
}

Answer 1

只需添加$match管道阶段即可跳过包含空inventory_docs字段的文档。没有其他方法可以实现这一点。

查询：

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

结果：

{
    "_id" : 1.0,
    "sku" : "abc",
    "description" : "product 1",
    "instock" : 120.0,
    "inventory_docs" : [ 
        {
            "_id" : 1.0,
            "item" : "abc",
            "price" : 12.0,
            "quantity" : 2.0
        }
    ]
}

{
    "_id" : 4.0,
    "sku" : "jkl",
    "description" : "product 4",
    "instock" : 70.0,
    "inventory_docs" : [ 
        {
            "_id" : 2.0,
            "item" : "jkl",
            "price" : 20.0,
            "quantity" : 1.0
        }
    ]
}

如何在MongoDB聚合中使用$ lookup作为INNER JOIN？

1 个答案: