How to Make a Lookup connection between two Collection - sql

Question

Goal:
This sql and its result should be the same result from mongoDB's query code.
In order words, same result but for mongoDB.
Problem:
How to you make a lookup connection in relation to People and Role in Mongo DB's query code?
Info:
I'm new in mongo DB
SQL code
SELECT
a.*,
'.' AS '.',
b.*,
'.' AS '.',
c.*
FROM
[db1].[dbo].[People_Course_Grade] a
INNER JOIN [db1].[dbo].[People] b on a.PeopleId = b.PeopleId
INNER JOIN [db1].[dbo].[Role] c on b.RoleId = c.RoleId
Json data:
Role:
[{"RoleId":1,"Name":"Student"},{"RoleId":2,"Name":"Teacher"}]
People_Course_Grade:
[{"People_Course_GradeId":1,"PeopleId":1,"CourseId":1},
{"People_Course_GradeId":2,"PeopleId":2,"CourseId":1},
{"People_Course_GradeId":3,"PeopleId":3,"CourseId":2},
{"People_Course_GradeId":4,"PeopleId":1,"CourseId":2}]
Course:
[{"CourseId":1,"Name":"Java"},{"CourseId":2,"Name":"Java II"},
{"CourseId":3,"Name":"Statistik 1"}]
db.People_Course_Grade.aggregate([
{
$lookup:{
from: "People",
localField: "people_id",
foreignField: "_id",
as: "people"
}
},
{ $unwind:"$people" },
{
$project:{
course_id : 1,
people_id : 1,
// grade_id : 1,
Name : "$people.Name",
}
}
]);

Answer 1

You need to start with double $lookup since you have three collections. Then you can use $arrayElemAt to always get single element from lookup's result. To flatten your structure you can use $replaceRoot with $mergeObjects (promotes all the fields from people and course to root level.
db.People_Course_Grade.aggregate([
{
$lookup:{
from: "Role",
localField: "PeopleId",
foreignField: "RoleId",
as: "people"
}
},
{
$lookup:{
from: "Course",
localField: "CourseId",
foreignField: "CourseId",
as: "course"
}
},
{
$replaceRoot: {
newRoot: {
$mergeObjects: [
"$$ROOT",
{ $arrayElemAt: [ "$people", 0 ] },
{ $arrayElemAt: [ "$course", 0 ] },
]
}
}
},
{
$project: {
people: 0,
course: 0
}
}
])
Mongo Playground
$arrayElemAt can always be replaced with $unwind like you tried. You also have a naming conflict on name field so probably you need to run $project to rename one of those fields - otherwise you'll get only one of them in final result.