SpringData MongoDb, how to count distinct of a query? - mongodb

Question

I'm doing paginated search with mongoDb in my Springboot API.
For a customer search path, I'm building a query with a bunch of criteria depending on the user input.
I then do a count to display the total number of results (and the computed number of page associated)
Long total = mongoTemplate.count(query, MyEntity.class);
I then do the paginated query to return only current page results
query.with(PageRequest.of(pagination.getPage(), pagination.getPageSize()));
query.with(Sort.by(Sort.Direction.DESC, "creationDate"));
List<MyEntity> listResults = mongoTemplate.find(query, MyEntity.class);
It all works well.
Now on my total results, i often have multiple result for the same users, I want to display those in the paginated list, but I also want to display a new counter with the total distinct user that are in that search.
I saw the findDistinct parameter
mongoTemplate.findDistinct(query, "userId", OnboardingItineraryEntity.class, String.class);
But I do not want to retrieve a huge list and do a count on it. Is there a way to easily do:
mongoTemplate.countDistinct(query, "userId", OnboardingItineraryEntity.class, String.class);
Cause I've a huge number of criteria, so i find it sad to have to rebuild an Aggregate object from scratch ?
Bonus question, sometime userId will be null, Is there an easy way do count number of distinct (not null) + number of null in one query?
Or do I need to do a query, when i add an extra criteira on userId being null, do a count on that, and then do the count distinct on all and add them up manualy in my code (minus one).

Answer 1

MongoDB aggregation solves this problem in several ways.
Aggregate with $type operator:
db.myEntity.aggregate([
{$match:...}, //add here MatchOperation
{
"$group": {
"_id": {
"$type": "$userId"
},
"count": {
"$sum": 1
}
}
}
])
MongoPlayground
---Ouput---
[
{
"_id": "null", //null values
"count": 2
},
{
"_id": "missing", // if userId doesn't exists at all
"count": 1
},
{
"_id": "string", //not null values
"count": 4
}
]
Single document with null and NonNull fields
db.myEntity.aggregate([
{$match:...}, //add here MatchOperation
{
"$group": {
"_id": "",
"null": {
$sum: {
$cond: [
{
$ne: [{ "$type": "$userId"}, "string"]
},
1,
0
]
}
},
"nonNull": {
"$sum": {
$cond: [
{
$eq: [{ "$type": "$userId" }, "string"]
},
1,
0
]
}
}
}
}
])
MongoPlayground
---Output---
[
{
"_id": "",
"nonNull": 4,
"null": 3
}
]
Performing $facet operator
db.myEntity.aggregate([
{$match:...}, //add here MatchOperation
{
$facet: {
"null": [
{
$match: {
$or: [
{
userId: {
$exists: false
}
},
{
userId: null
}
]
}
},
{
$count: "count"
}
],
"nonNull": [
{
$match: {
$and: [
{
userId: {
$exists: true
}
},
{
userId: {
$ne: null
}
}
]
}
},
{
$count: "count"
}
]
}
},
{
$project: {
"null": {
$ifNull: [
{
$arrayElemAt: [
"$null.count",
0
]
},
0
]
},
"nonNull": {
$ifNull: [
{
$arrayElemAt: [
"$nonNull.count",
0
]
},
0
]
}
}
}
])
MongoPlayground
Note: Try any of these solutions and let me know if you have any problem creating the MongoDB aggregation.