如何在mongoDB中对$ lookup结果应用条件？

Question

在我之前的问题的参考,我有一个关于$ lookup的问题,添加一些条件.您可以从以下链接描述中获得有关问题的足够参考.

照片:

{_id: 1, photo_name: '1.jpg', photo_description: 'description 1', album_id: 1, flag:1 },
{_id: 2, photo_name: '2.jpg', photo_description: 'description 2', album_id: 1, flag:1 },
{_id: 3, photo_name: '3.jpg', photo_description: 'description 3', album_id: 1, flag:1 },
{_id: 4, photo_name: '4.jpg', photo_description: 'description 4', album_id: 2, flag:0 },
{_id: 5, photo_name: '5.jpg', photo_description: 'description 5', album_id: 2, flag:0 },
{_id: 6, photo_name: '6.jpg', photo_description: 'description 6', album_id: 2}

专辑:

{_id: 1, album_name: "my album 1", album_description: "album description 1", emoji_id: 1},
{_id: 2, album_name: "my album 2", album_description: "album description 2", emoji_id: 2},
{_id: 3, album_name: "my album 3", album_description: "album description 3", emoji_id: 3},
{_id: 4, album_name: "my album 4", album_description: "album description 4", emoji_id: 4},
{_id: 5, album_name: "my album 5", album_description: "album description 5", emoji_id: 5}

表情:

{_id: 1, emoji_name: "1.jpg"},  
{_id: 2, emoji_name: "2.jpg"},  
{_id: 3, emoji_name: "3.jpg"},  
{_id: 4, emoji_name: "4.jpg"},  
{_id: 5, emoji_name: "5.jpg"},  
{_id: 6, emoji_name: "6.jpg"},  
{_id: 7, emoji_name: "7.jpg"},  
{_id: 8, emoji_name: "8.jpg"}   

测试记录分页:

2

假设我在照片集中添加了另一个标志字段,现在我想只计算那些标志为1的照片.

我试图在$ lookup中查询后立即添加$ match,但它失败了,它不会排除其flag = 0的照片,也不会排除计数器中没有标记条件的照片.

当前输出10张照片中有3张照片设置了标志0.我们无法考虑那些标志为0的照片.所以预计总共有7张照片,但是虽然我在照片中应用了旗帜状态,但是计数会返回10张照片.

现在的查询:

 db
.album
.aggregate([
  {
     $lookup:{
        from:"photo",
        localField:"_id",
        foreignField:"album_id",
        as:"photo"
     }
  },
  {
     $match:{
        "photo.flag": 1
     }
  },
  {
     $lookup:{
        from:"emoji",
        localField:"album_emoji",
        foreignField:"_id",
        as:"emoji"
     }
  },
  {
     $project:{
        album_name:"$album_name",
        album_description:"$album_description",
        album_emoji:"$emoji.image_name",
        photo:"$photo",
        total_photos: {$size: "$photo"}
     }
  }
])
.toArray();

预期产量:

[
    {
        "_id" : 1,
        "album_name" : "Album 1",
        "album_description" : "Album description 1",
        "album_emoji" : [
            "1.jpg"
        ],
        "total_photos" : 3,
        "photo" : [
            {
                "_id" : 1,
                "album_id" : 1,
                "photo_description" : "description 1",
                "photo_name" : "1.jpg",             
                "flag" : 0,
            },
            {
                "_id" : 2,
                "album_id" : 1,
                "photo_description" : "description 2",
                "photo_name" : "2.jpg",             
                "flag" : 0,
            },
            {
                "_id" : 1,
                "album_id" : 1,
                "photo_description" : "description 3",
                "photo_name" : "3.jpg",             
                "flag" : 0,
            }
        ]
    }
]

目前的输出:

[
    {
        "_id" : 1,
        "album_name" : "Album 1",
        "album_description" : "Album description 1",
        "album_emoji" : [
            "1.jpg"
        ],
        "total_photos" : 5,
        "photo" : [
            {
                "_id" : 1,
                "album_id" : 1,
                "photo_description" : "description 1",
                "photo_name" : "1.jpg",             
                "flag" : 1,
            },
            {
                "_id" : 2,
                "album_id" : 1,
                "photo_description" : "description 2",
                "photo_name" : "2.jpg",             
                "flag" : 1,
            },
            {
                "_id" : 3,
                "album_id" : 1,
                "photo_description" : "description 3",
                "photo_name" : "3.jpg",             
                "flag" : 1,
            },
            {
                "_id" : 4,
                "album_id" : 1,
                "photo_description" : "description 4",
                "photo_name" : "4.jpg",             
                "flag" : 0,
            },
            {
                "_id" : 5,
                "album_id" : 1,
                "photo_description" : "description 5",
                "photo_name" : "5.jpg",             
                "flag" : 0,
            }
        ]
    }
]

Answer 1

您不能在“$lookup”之后将“$match”与对象方法一起使用，因为“$lookup”的返回值是数组值。您最好在查找后添加“$unwind”函数，然后对其进行分组。

示例查询

 db
.album
.aggregate([
    {
        $lookup:{
            from:"photo",
            localField:"_id",
            foreignField:"album_id",
            as:"photo"
        }
     },
    {
        preserveNullAndEmptyArrays : true,
        path : "$photo"
    },
    {
        $match:{
            "photo.flag": 1
        }
     },
     {
        $group : {
            _id : {
                id : "$_id",
                album_name: "$album_name",
                album_description: "$album_description",
                emoji_id: "$emoji_id"
            },
            photo: {
                $push : "$photo"
            }
        }
     }
    {
         $lookup:{
            from:"emoji",
            localField:"_id.album_emoji",
            foreignField:"_id",
            as:"emoji"
         }
    },
    {
         $project:{
            album_name:"$album_name",
            album_description:"$album_description",
            emoji:"$emoji",
            photo:"$photo",
            total_photos: {$size: "$photo"}
         }
    }
])

或者使用“$filter”。

db
.album
.aggregate([
    {
        $lookup:{
            from:"photo",
            localField:"_id",
            foreignField:"album_id",
            as:"photo"
        }
     },
    {
        $project: {
            id : "$_id",
            album_name: "$album_name",
            album_description: "$album_description",
            emoji_id: "$emoji_id",
            photo: {
                $filter : {
                    input: "$photo",
                    as : "photo_field",
                    cond : {
                        $eq: ["$$photo_field.flag",1]
                    }
                }
            }
        }
    },
    {
         $lookup:{
            from:"emoji",
            localField:"album_emoji",
            foreignField:"_id",
            as:"emoji"
         }
    },
    {
         $project:{
            album_name:"$album_name",
            album_description:"$album_description",
            emoji:"$emoji",
            photo:"$photo",
            total_photos: {$size: "$photo"}
         }
    }
])