Querying, projecting and sorting multiple fields of documents inside a mongoDB collection - mongodb

Question

I have a simple mongoDB collection, named usersCollection, whose generale structure is this:
[
{
"username" : "john",
"sex" : "m",
"email" : "john#gmail.com",
"courses" : [
{
"title" : "medicine",
"grade": 25,
},
{
"title" : "art",
"grade": 29,
},
{
"title" : "history",
"grade": 21,
}
]
},
{
"username" : "jane",
"sex" : "f",
"email" : "jane#gmail.com",
"courses" : [
{
"title" : "math",
"grade": 20,
},
{
"title" : "medicine",
"grade": 30,
}
]
},
{
"username" : "sarah",
"sex" : "f",
"email" : "sarah#gmail.com",
"courses" : [ ]
},
{
"username" : "josh",
"sex" : "f",
"email" : "josh#gmail.com",
"courses" : [
{
"title" : "english",
"grade": 28,
}
]
},
{
"username" : "mark",
"sex" : "m",
"email" : "mark#gmail.com",
"courses" : [
{
"title" : "history",
"grade": 30,
},
{
"title" : "medicine",
"grade": 19,
},
{
"title" : "math",
"grade": 22,
}
]
}
]
Every user is a member of usersCollection and has some general informations and a list of courses that he/she has already completed with the relative grades.
Can anyone tell me how I can query usersCollection to get a descending sorted array that contains an object for all users that have already completed a specific course? Every object of this array should contains the "name", the "email" and the "grade" of the relative user.
For example, launching a query on usersCollection around the course with name "medicine", I would obtain this array:
[
{
"username": "jane",
"email": "jane#gmail.com",
"grade": 30
},
{
"username": "john",
"email": "john#gmail.com",
"grade": 25
},
{
"username": "mark",
"email": "mark#gmail.com",
"grade": 19
}
]

Answer 1

You can manage to do it using sort and group pipeline of mongoDB.
if you want to get any specific courses you can apply match pipeline before sort pipeline.
db.users.aggregate([
{
$unwind: "$courses"
},
{
$match: {
'courses.title': 'medicine'
}
},
{
$sort: {
'courses.title': -1,
'courses.grade': -1,
}
},
{
$group: {
_id: "$courses.title",
records: {
$push: {
"_id" : "$_id",
"username" : "$username",
"sex" : "$sex",
"email" : "$email",
"grade":"$courses.grade"
}
}
}
}
])