Develop Reference

How to parse MongoDB aggregation step containing functions?

Looking at com.mongodb.reactivestreams.client.MongoCollection interface we see that aggregation can be invoked using list of Bson elements.
public interface MongoCollection<TDocument> {
...
AggregatePublisher<TDocument> aggregate(List<? extends Bson> list);
}
It is clear how to use it when aggregation steps are JSONs (see Example with simple JSONs)
Unfortunately, when any aggregation step contains a function (which is allowed by native MongoDB query), for instance, $accumulator the same approach cannot be applied due to it causes violation of Bson format (org.bson.json.JsonParseException) (see Example with functions)
What is the best way to convert a native MongoDB aggregation query into a result in Java?
(suppose that queries are complex and it is not expedient to rewrite them with Mongo aggregation builders in Java)
Example with simple JSONs:
ReactiveMongoOperations mongo = /* ... */;
var match = BasicDBObject.parse("{ $match: {name: \"Jack\"} }");
var project = BasicDBObject.parse("{ $project: {_id: 0, age: 1, name: 1} }";
var queryParts = List.of(match, project);
Flux<PersonInfo> infoFlux = mongo
.getCollection("person")
.flatMapMany(person -> person.aggregate(queryParts).toFlux())
.map(it -> objectMapper.readValue(it.toJson(), PersonInfo.class))
.collectList()
Example with functions:
// here for conciseness it is just a counting accumulator; generally functions are more complex
var match = BasicDBObject.parse("""
{
$group: {
_id: "$token",
count: {
$accumulator: {
init: function() {
return {owned: 0, total: 0}
},
accumulate: function(state, owner) {
return {
total: state.total + 1
}
},
accumulateArgs: ["$owner"],
merge: function(a, b) {
return {
total: a.total + b.total
}
},
lang: "js"
}
},
minPriceEth: {$min: "$priceEth"}
}
}
""");

Spring Data Mongo Aggregation Function with both $trim and $toLower

Trying to convert the following mongo query but cant find a solution to trim and toLower using Spring Aggregation queries
db.qabr.aggregate([
{ $match: {name: { "$ne": '' } }},
{ $group: {
_id: { 'name': { input: { $trim: { input: { $toLower : '$name' }}}}, 'qabrNumber': '$qabrNumber', 'qabristaan': '$qabristaan' }, // can be grouped on multiple properties
count: { "$sum": 1 }
}},
{ $match: {
count: { "$gt": 1 } // Duplicates considered as count greater than one
}
}
]
)
If I do the following:
AddFieldsOperation trimAndLowerName = Aggregation.addFields()
.addFieldWithValue("name", StringOperators.Trim.valueOf(
StringOperators.ToLower.lowerValueOf("name")
))
.build();
I keep getting 168 (InvalidPipelineOperator): 'Unrecognized expression '$trim'' on server
However running just trim or just toLower works:
AddFieldsOperation trimName = Aggregation.addFields()
.addFieldWithValue("name", StringOperators.ToLower.lowerValueOf("name"))
.build();

How to do this aggregation in Spring Data MongoDB?

I'm not used to work with Spring Data and I'm trying to do this MongoDB aggregation but I'm not able to solve the project and group part, match part was pretty easy:
db.collection.aggregate(
{ $match: { "car._id": "abc1234" } },
{
$project: {
month: { $month: "$day" },
year: { $year: "$day" },
services: 1
}
},
{
$group: {
_id: { month: "$month", year: "$year" },
total: { $sum: "$services" }
}
}
)
day is a Date type field. The query is working fine on the mongo shell, filtering by _id and grouping by year and months with the sum of all services (Int field). But I'm not able to implement it on Spring Data MongoDB.
I've tried with the Aggregation.group() but I'm getting lost because of the nested object in the _id.

as far as I know you must wrap everything in an array like so:
db.collection.aggregate([
{ $match: { "car._id": "abc1234" } },
{
$project: {
month: { $month: "$day" },
year: { $year: "$day" },
services: 1
}
},
{
$group: {
_id: { month: "$month", year: "$year" },
total: { $sum: "$services" }
}
}])

Creating Spring Data Aggregation of multiple MongoDB queries

The database MongoDB I have stored documents in the format:
{
"achievement": [
{
"userFromId":"max",
"userToId":"peter",
"date":"2016-01-25",
"pointCount":1,
"description":"good work",
"type":"THANKS"
}
]
}
How to get the number of records in the database (if any) for the a certain date, in which people are thanking the other people.
I created a query to retrieve data:
DBObject clause1 = new BasicDBObject("userFromId", userFromId);
DBObject clause2 = new BasicDBObject("userToId", userToId);
DBObject clause3 = new BasicDBObject("sendDate", localDate);
DBObject clause4 = new BasicDBObject("type", Thanks);
BasicDBList or = new BasicDBList();
or.add(clause1);
or.add(clause2);
or.add(clause3);
or.add(clause4);
DBObject query = new BasicDBObject("$or", or);
But I do not know how to get the number of records and how can rewrite the query using aggregation?
For example:
Aggregation aggregation = Aggregation.newAggregation(
Aggregation.group("userFromId")
.first("userFromId").as("userFromId")
.sum("pointCount").as("pointCount"));
I do not know how to add a few more parameters.
What the return request if the data to the database does not exist?
Thanks for any help

You can use something like this. This will count all the number of documents matching the below criteria.
Regular Query
db.collection.count({ $or: [ { "userFromId": userFromId }, { "userToId": userToId } ] });
Using Aggregation
db.collection.aggregate( [
{ $match: { $or: [ { "userFromId": userFromId }, { "userToId": userToId } ] } },
{ $group: { _id: null, count: { $sum: 1 } } }
] );

java mongodb - get array length without downloading all data

I'm using mongodb to store data for my java program and I have a collection with an array field that has a lot of things in it but i want only to get the length, without all the other data.
Now i'm using this to get it:
((UUID[])document.get("customers")).length
How can I make this not to download all the array?
A possible answer is to create an int that counts the pushes and the pulls of the array but it's not the cleanest method.

You are looking for aggregation framework where you can use the $size operator in your pipeline, this counts and returns the total the number of items in an array:
db.collection.aggregate([
{
"$project": {
"_id": 0, "customer_count": { "$size": "$customers" }
}
}
]);
where the Java equivalent:
DBObject projectFields = new BasicDBObject("_id", 0);
projectFields.put("customer_count", new BasicDBObject( "$size", "$customers" ));
DBObject project = new BasicDBObject("$project", projectFields);
AggregationOutput output = db.getCollection("collectionName").aggregate(project);
System.out.println("\n" + output);

You can use MongoDB's Aggregation Framework to get the size of the array. For example, given the following document structure:
> db.macross.findOne()
{
"_id" : "SDF1",
"crew" : [
"Rick",
"Minmay",
"Roy",
"Max",
"Misa",
"Milia"
]
}
get the size of the array
> db.macross.aggregate(
{ $match: { _id: "SDF1" } },
{ $unwind: "$crew" },
{ $group: { _id: "", count: { $sum: 1 } } },
{ $project: { _id: 0, count: 1 } }
)
{ "count" : 6 }
More detailed and interesting examples are available in the docs.