Get informations from 2 models with Mongoose/NodeJs [duplicate] - node.js

Question

I have a collection called article_category which store all article_id belongs to the category with category_id with data format like so.
Collection 1: article_category
{
"article_id": 2015110920343902,
"all_category_id": [5,8,10]
}
Then I have other collection called article which store all my post
Collection 2: article
{
"title": "This is example rows in article collection"
"article_id": 2015110920343902,
},
{
"title": "Something change"
"article_id": 2015110920343903,
},
{
"title": "This is another rows",
"article_id": 2015110920343904,
}
Now I want to perform MongoDB query to find title with regex while category_id must equal to 8. Here is my query but is not work.
db.article.aggregate(
{
$match:
{
title:
{
$regex: /example/
}
}
},
{
$lookup:
{
from: "article_category",
pipeline: [
{ $match: { category_id: 8 } }
],
as: "article_category"
}
}
)
Above query only show the records which match by regex but not match by category_id.
Any idea?

Answer 1

First of all, it is all_category_id, not category_id. Secondly, you don't link articles - all documents will have exactly the same article_category array. Lastly, you probably want to filter out articles that don't have matched category. The conditional pipeline should look more like this:
db.article.aggregate([
{ $match: {
title: { $regex: /example/ }
} },
{ $lookup: {
from: "article_category",
let: {
article_id: "$article_id"
},
pipeline: [
{ $match: {
$expr: { $and: [
{ $in: [ 8, "$all_category_id" ] },
{ $eq: [ "$article_id", "$$article_id" ] }
] }
} }
],
as: "article_category"
} },
{ $match: {
$expr: { $gt: [
{ $size: "$article_category"},
0
] }
} }
] )
UPDATE:
If you don't match article_id, the $lookup will result with identical article_category array to all articles.
Let's say your article_category collection has another document:
{
"article_id": 0,
"all_category_id": [5,8,10]
}
With { $eq: [ "$article_id", "$$article_id" ] } in the pipeline the resulting article_category is
[
{
"article_id" : 2015110920343902,
"all_category_id" : [ 5, 8, 10 ]
}
]
without:
[
{
"article_id" : 2015110920343902,
"all_category_id" : [ 5, 8, 10 ]
},
{
"article_id": 0,
"all_category_id": [ 5, 8, 10 ]
}
]
If the later is what you need, it would be way simpler to make to find requests:
db.article.find({ title: { $regex: /example/ } })
and
db.article_category.find({ all_category_id: 8 })

Answer 2

You've couple of things incorrect here. category_id should be all_category_id. Use the join condition in $lookup and move the $match outside of $lookup stage with $unwind for optimized lookup.
Use $project with exclusion to drop the looked up field from final response.
Something like {$project:{article_category:0}}
Try
db.article.aggregate([
{"$match":{"title":{"$regex":/example/}}},
{"$lookup":{
"from":"article_category",
"localField":"article_id",
"foreignField":"article_id",
"as":"article_category"
}},
{"$unwind":"$article_category"},
{"$match":{"article_category.all_category_id":8}}
])
For uncorrelated subquery try
db.article.aggregate([
{"$match":{"title":{"$regex":/example/}}},
{"$lookup":{
"from":"article_category",
"pipeline":[{"$match":{"all_category_id":8}}],
"as":"categories"
}},
{"$match":{"categories":{"$ne":[]}}}
])