Aggregate on array of embedded documents - mongodb

Question

I have a mongodb collection with multiple documents. Each document has an array with multiple subdocuments (or embedded documents i guess?). Each of these subdocuments is in this format:
{
"name": string,
"count": integer
}
Now I want to aggregate these subdocuments to find
The top X counts and their name.
Same as 1. but the names have to match a regex before sorting and limiting.
I have tried the following for 1. already - it does return me the top X but unordered, so I'd have to order them again which seems somewhat inefficient.
[{
$match: {
_id: id
}
}, {
$unwind: {
path: "$array"
}
}, {
$sort: {
'count': -1
}
}, {
$limit: x
}]
Since i'm rather new to mongodb this is pretty confusing for me. Happy for any help. Thanks in advance.

Answer 1

The sort has to include the array name in order to avoid an additional sort later on.
Given the following document to work with:
{
students: [{
count: 4,
name: "Ann"
}, {
count: 7,
name: "Brad"
}, {
count: 6,
name: "Beth"
}, {
count: 8,
name: "Catherine"
}]
}
As an example, the following aggregation query will match any name containing the letters "h" and "e". This needs to happen after the "$unwind" step in order to only keep the ones you need.
db.tests.aggregate([
{$match: {
_id: ObjectId("5c1b191b251d9663f4e3ce65")
}},
{$unwind: {
path: "$students"
}},
{$match: {
"students.name": /[he]/
}},
{$sort: {
"students.count": -1
}},
{$limit: 2}
])
This is the output given the above mentioned input:
{ "_id" : ObjectId("5c1b191b251d9663f4e3ce65"), "students" : { "count" : 8, "name" : "Catherine" } }
{ "_id" : ObjectId("5c1b191b251d9663f4e3ce65"), "students" : { "count" : 6, "name" : "Beth" } }
Both names contain the letters "h" and "e", and the output is sorted from high to low.
When setting the limit to 1, the output is limited to:
{ "_id" : ObjectId("5c1b191b251d9663f4e3ce65"), "students" : { "count" : 8, "name" : "Catherine" } }
In this case only the highest count has been kept after having matched the names.
=====================
Edit for the extra question:
Yes, the first $match can be changed to filter on specific universities.
{$match: {
university: "University X"
}},
That will give one or more matching documents (in case you have a document per year or so) and the rest of the aggregation steps would still be valid.
The following match would retrieve the students for the given university for a given academic year in case that would be needed.
{$match: {
university: "University X",
academic_year: "2018-2019"
}},
That should narrow it down to get the correct documents.