Getting aggregate count for nested fields in mongo - mongodb

Question

I have a mongo collection mimicking this java class. A student can be taught a number of subjects across campus.
class Students {
String studentName;
Map<String,List<String>> subjectsByCampus;
}
So a structure will look like this
{
_id: ObjectId("someId"),
studentName:'student1',
subjectByCampusName:{
campus1:['subject1','subject2'],
campus2: ['subject3']
},
_class: 'fqnOfTheEntity'
}
I want to find the count of subjects offered by each campus or be able to query the count of subjects offered by a specific campus. Is there a way to get it through query?

Answer 1

Mentioned in the comments, but the schema here does not appear to be particularly well-suited toward gathering the data requested in the question. For an individual student, this is doable via $size and processing the object (as an array) via $map. For example, if we want your desired out put of campus1:2, campus2:1 for the sample document provided, a pipeline to produce that in a countsByCampus field might look as follows:
[
{
"$addFields": {
"countsByCampus": {
"$arrayToObject": {
"$map": {
"input": {
"$objectToArray": "$subjectByCampusName"
},
"in": {
"$mergeObjects": [
"$$this",
{
v: {
$size: "$$this.v"
}
}
]
}
}
}
}
}
}
]
Playground demonstration here with an output document of:
{
"_class": "fqnOfTheEntity",
"_id": "someId",
"countsByCampus": {
"campus1": 2,
"campus2": 1
},
"studentName": "student1",
"subjectByCampusName": {
"campus1": [
"subject1",
"subject2"
],
"campus2": [
"subject3"
]
}
}
Doing that across the entire collection would involve $grouping the results together. This can be done but would be an extremely resource-intensive and likely slow operation.