MongoDB inner join with specific condition from both collections - mongodb

Question

I have got two collections, chapters and courses with one-to-one association,
db={
"chapters": [
{
"_id": 10,
"course_id": 1,
"author": "John"
},
{
"_id": 20,
"course_id": 2,
"author": "John"
},
{
"_id": 30,
"course_id": 3,
"author": "Timmy"
},
{
"_id": 40,
"course_id": 4,
"author": "John"
},
],
"courses": [
{
"_id": 1,
"published": true,
"name": "course 1"
},
{
"_id": 2,
"published": false,
"name": "course 2"
},
{
"_id": 3,
"published": true,
"name": "course 3"
},
{
"_id": 4,
"published": true,
"name": "course 4"
}
]
}
How do I query all chapters with the published course (published=true) and the author is "John"?

Answer 1

You can use $match, $lookup then $group by author then simply $filter it based on published
db.chapters.aggregate([
{
$match: {
author: "John"
}
},
{
"$lookup": {
"from": "courses",
"localField": "course_id",
"foreignField": "_id",
"as": "course"
}
},
{
$project: {
_id: 1,
course: {
$first: "$course"
},
author: 1
}
},
{
$group: {
_id: "$author",
courseChapters: {
$push: "$$ROOT"
}
}
},
{
$project: {
courseChapters: {
$filter: {
input: "$courseChapters",
as: "cc",
cond: {
$eq: [
"$$cc.course.published",
true
]
}
}
}
}
}
])
Output
[
{
"_id": "John",
"courseChapters": [
{
"_id": 10,
"author": "John",
"course": {
"_id": 1,
"name": "course 1",
"published": true
}
},
{
"_id": 40,
"author": "John",
"course": {
"_id": 4,
"name": "course 4",
"published": true
}
}
]
}
]
Mongoplayground: https://mongoplayground.net/p/x-XghXzZUk4