本文介绍了如何在mongoDB中对$ lookup结果应用条件?的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
参考我先前的问题，我对$ lookup有一个疑问，并添加了一些条件.您可以从下面的链接描述中获得有关问题的足够参考.
With the reference of my previous question, I have a question about $lookup with add some conditions. You may get enough reference about question from below link description.
{_id: 1, photo_name: '1.jpg', photo_description: 'description 1', album_id: 1, flag:1 },
{_id: 2, photo_name: '2.jpg', photo_description: 'description 2', album_id: 1, flag:1 },
{_id: 3, photo_name: '3.jpg', photo_description: 'description 3', album_id: 1, flag:1 },
{_id: 4, photo_name: '4.jpg', photo_description: 'description 4', album_id: 2, flag:0 },
{_id: 5, photo_name: '5.jpg', photo_description: 'description 5', album_id: 2, flag:0 },
{_id: 6, photo_name: '6.jpg', photo_description: 'description 6', album_id: 2}
相册:
{_id: 1, album_name: "my album 1", album_description: "album description 1", emoji_id: 1},
{_id: 2, album_name: "my album 2", album_description: "album description 2", emoji_id: 2},
{_id: 3, album_name: "my album 3", album_description: "album description 3", emoji_id: 3},
{_id: 4, album_name: "my album 4", album_description: "album description 4", emoji_id: 4},
{_id: 5, album_name: "my album 5", album_description: "album description 5", emoji_id: 5}
表情符号:
{_id: 1, emoji_name: "1.jpg"},
{_id: 2, emoji_name: "2.jpg"},
{_id: 3, emoji_name: "3.jpg"},
{_id: 4, emoji_name: "4.jpg"},
{_id: 5, emoji_name: "5.jpg"},
{_id: 6, emoji_name: "6.jpg"},
{_id: 7, emoji_name: "7.jpg"},
{_id: 8, emoji_name: "8.jpg"}
测试记录分页:
2
假设我在照片收藏夹中添加了另外一个标志字段，现在我只想计数那些标志为1的照片.
Suppose I add one another field of flag in photo collection and now I want to get count only those photos whose flag is one.
我试图在查询中在$ lookup之后立即添加$ match，但是失败了，它不排除flag = 0的照片，并且在计数器中它不标记条件.
I tried to add $match immediately after $lookup in query, but it fails, It doesn't exclude photos whose flag=0 and also in counter it does not flag condition.
当前查询:
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: "$photo"}
}
}
])
.toArray();
[
{
"_id" : 1,
"album_name" : "Album 1",
"album_description" : "Album description 1",
"album_emoji" : [
"1.jpg"
],
"total_photos" : 3,
"photo" : [
{
"_id" : 1,
"album_id" : 1,
"photo_description" : "description 1",
"photo_name" : "1.jpg",
"flag" : 0,
},
{
"_id" : 2,
"album_id" : 1,
"photo_description" : "description 2",
"photo_name" : "2.jpg",
"flag" : 0,
},
{
"_id" : 1,
"album_id" : 1,
"photo_description" : "description 3",
"photo_name" : "3.jpg",
"flag" : 0,
}
]
}
]
[
{
"_id" : 1,
"album_name" : "Album 1",
"album_description" : "Album description 1",
"album_emoji" : [
"1.jpg"
],
"total_photos" : 5,
"photo" : [
{
"_id" : 1,
"album_id" : 1,
"photo_description" : "description 1",
"photo_name" : "1.jpg",
"flag" : 1,
},
{
"_id" : 2,
"album_id" : 1,
"photo_description" : "description 2",
"photo_name" : "2.jpg",
"flag" : 1,
},
{
"_id" : 3,
"album_id" : 1,
"photo_description" : "description 3",
"photo_name" : "3.jpg",
"flag" : 1,
},
{
"_id" : 4,
"album_id" : 1,
"photo_description" : "description 4",
"photo_name" : "4.jpg",
"flag" : 0,
},
{
"_id" : 5,
"album_id" : 1,
"photo_description" : "description 5",
"photo_name" : "5.jpg",
"flag" : 0,
}
]
}
]
推荐答案
在"$ lookup"之后不能将"$ match"与对象方法一起使用，因为"$ lookup"的返回值是数组值.您最好在查询后添加"$ unwind"功能，然后将其分组.
You can't use "$match" with a object method after "$lookup", because return value of "$lookup" are array values. You better add "$unwind" function after the look up then group it.
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: "$photo"}
}
}
])
或使用"$ filter".
Or use "$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: "$photo"}
}
}
])
这篇关于如何在mongoDB中对$ lookup结果应用条件?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！