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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.
Photo:
{_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}
Album:
{_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}
Emoji:
{_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"}
Testing record pagination :
2
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.
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.
Present Output There are 3 photos out of 10 photos having set flag 0. And we could not consider those photos whose flag is 0. So expected total is 7 photos but count returns 10 photos though I applied condition of flag in photos.
Present Query:
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();
Expected output:
[
{
"_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,
}
]
}
]
Present output:
[
{
"_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,
}
]
}
]
814
The $lookup operator is an aggregation operator or an aggregation stage, which is used to join a document from one collection to a document of another collection of the same database based on some queries. Both the collections should belong to the same databases.
For performing MongoDB Join two collections, you must use the $lookup operator. It is defined as a stage that executes a left outer join with another collection and aids in filtering data from joined documents. For example, if a user requires all grades from all students, then the below query can be written: Students.
Basically, MongoDB provides the different match operators such as $match and $count, etc, to the user and we can utilize them as per our requirement. We can also use a match operator for the aggregation pipeline. In the above syntax, we use the $match Mongodb operator as shown.
$merge inserts the document directly into the output collection. Prior to MongoDB 4.2. 2, when these conditions for a $merge stage are met, the pipeline specified in the whenMatched field is executed with an empty input document. The resulting document from the pipeline is inserted into the output collection.
You can't use "$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.
Example Query
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: "$photo"}
}
}
])
Or use "$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"}
}
}
])
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