Get document by min size of array in Mongodb - spring

Question

I have mongo collection:
{
"_id" : 123,
"index" : "111",
"students" : [
{
"firstname" : "Mark",
"lastname" : "Smith"),
}
],
}
{
"_id" : 456,
"index" : "222",
"students" : [
{
"firstname" : "Mark",
"lastname" : "Smith"),
}
],
}
{
"_id" : 789,
"index" : "333",
"students" : [
{
"firstname" : "Neil",
"lastname" : "Smith"),
},
{
"firstname" : "Sofia",
"lastname" : "Smith"),
}
],
}
I want to get document that has index that is in the set of the given indexes, for example givenSet = ["111","333"] and has min length of students array.
Result should be the first document with _id:123, because its index is in the givenSet and studentsArrayLength = 1, which is smaller than third.
I need to write custom JSON #Query for Spring Mongo Repository. I am new to Mongo and am stuck a bit with this problem.
I wrote something like this:
#Query("{'index':{$in : ?0}, length:{$size:$students}, $sort:{length:-1}, $limit:1}")
Department getByMinStudentsSize(Set<String> indexes);
And got error: error message '$size needs a number'
Should I just use .count() or something like that?

Answer 1

you should use the aggregation framework for this type of query.
filter the result based on your condition.
add a new field and assign the array size to it.
sort based on the new field.
limit the result.
the solution should look something like this:
db.collection.aggregate([
{
"$match": {
index: {
"$in": [
"111",
"333"
]
}
}
},
{
"$addFields": {
"students_size": {
"$size": "$students"
}
}
},
{
"$sort": {
students_size: 1
}
},
{
"$limit": 1
}
])
working example: https://mongoplayground.net/p/ih4KqGg25i6

Answer 2

You are getting the issue because the second param should be enclosed in curly braces. And second param is projection
#Query("{{'index':{$in : ?0}}, {length:{$size:'$students'}}, $sort:{length:1}, $limit:1}")
Department getByMinStudentsSize(Set<String> indexes);
Below is the mongodb query :
db.collection.aggregate(
[
{
"$match" : {
"index" : {
"$in" : [
"111",
"333"
]
}
}
},
{
"$project" : {
"studentsSize" : {
"$size" : "$students"
},
"students" : 1.0
}
},
{
"$sort" : {
"studentsSize" : 1.0
}
},
{
"$limit" : 1.0
}
],
{
"allowDiskUse" : false
}
);