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how to give $toObjectId in Java Mongo Projection Operation

I am new bee to mongo. Below is the Aggregation operation I am doing in mongodb shell . But In my java ProjectionAggregation , am not able to give the $toObjectId. Please correct me what am I missing.
db shell query
db.getCollection('UserData').aggregate([
{
$project : {
"username" : "$username",
"beneficiaries" : "$beneficiaries"
}
},
{
$unwind : {
path : "$beneficiaries",
preserveNullAndEmptyArrays: true
}
},
{
$project : {
"username" : "$username",
"beneficiaries" : "$beneficiaries",
---- dont know how to give $toObjectId in java ProjectionOperation .
"beneficiaryStudId" : { $toObjectId : "$beneficiaries.studentId" }
}
},
{
$lookup:
{
from: "StudentProfileData",
localField: "beneficiaryStudId",
foreignField: "_id",
as: "studProfile"
}
}
])
Java code Projection Operation
ProjectionOperation projectUserAndBeneficiaries = Aggregation.project()
.andExpression("username").as("username")
.andExpression("beneficiaries").as("beneficiaries");
ProjectionOperation projectUserAndOtherDetails = Aggregation.project()
.andExpression("username").as("username")
.andExpression("beneficiaries").as("beneficiaries")
---- How to give $toObjectId in projection operation .andExpression("beneficiaries.studentId").as("beneficiaryStudId");
LookupOperation lookupOperation = LookupOperation.newLookup().
from("StudentProfileData").
localField("beneficiaryStudId").
foreignField("_id").
as("studProfile");
Aggregation agg = Aggregation.newAggregation(projectUserAndBeneficiaries, unwindBeneficiars,
projectUserAndOtherDetails
,lookupOperation);
AggregationResults<UserAndStudentData> output
= mongotemplate.aggregate(agg, "UserData", UserAndStudentData.class);
Sample Ouput
Output in db shell
{
"_id" : ObjectId("5d2f08574de2690001c281ac"),
"username" : "ks241#goo.com",
"beneficiaries" : {
"studentId" : "5d2f0e9c3bcf3e0001a7e562",
"mcBeneficiaryId" : "597418",
"enabled" : true
},
"beneficiaryStudId" : ObjectId("5d2f0e9c3bcf3e0001a7e562"),
"studProfile" : [
{
"_id" : ObjectId("5d2f0e9c3bcf3e0001a7e562"),
"lastName" : "Sharma",
"firstName" : "Kapil",
"studentRegisterCustomFieldValues" : [
{
"bcfdValue" : "One",
"bcfdName" : "Year"
}
],
"gender" : "M",
"merchantId" : "38788943"
}
]
}
where as in java
the studProfile array is always empty if I add $toObjectId the above java aggregation projection query and ran it produces the values as same as that db shell .

Spring data doesn't support to few type of methods. This problem may include into it. But we may use this solution.
Aggregation aggregation=newAggregation(
p-> new Document("$project",
new Document()
.append("username","$username"),
.append("beneficiaries","$beneficiaries)
.append("beneficiaryStudId",
new Document("$toObjectId","$beneficiaries.studentId")
)
)
)

There is a way to do this with Spring's ProjectionOperation and ConvertOperators classes:
ProjectionOperation projection = Aggregation.project()
.andInclude("username", "beneficiaries")
.and(ConvertOperators.ToObjectId.toObjectId("$beneficiaries.studentId"))
.as("beneficiaryStudId");

Get count of unique ObjectId from array MongoDB

I'm new to working with MongoDb and do not know a lot of things.
I need to write an aggregation request.
Here is the JSON document structure.
{
"_id" : ObjectId("5a72f7a75ef7d430e8c462d2"),
"crawler_id" : ObjectId("5a71cbb746e0fb0007adc6c2"),
"skill" : "stack",
"created_date" : ISODate("2018-02-01T13:19:03.522+0000"),
"modified_date" : ISODate("2018-02-01T13:22:23.078+0000"),
"connects" : [
{
"subskill" : "we’re",
"weight" : NumberInt(1),
"parser_id" : [
ObjectId("5a71d88d5ef7d41964fbec11")
]
},
{
"subskill" : "b1",
"weight" : NumberInt(2),
"parser_id" : [
ObjectId("5a71d88d5ef7d41964fbec11"),
ObjectId("5a71d88d5ef7d41964fbec1b")
]
},
{
"subskill" : "making",
"weight" : NumberInt(2),
"parser_id" : [
ObjectId("5a71d88d5ef7d41964fbec1b"),
ObjectId("5a71d88d5ef7d41964fbec1c")
]
},
{
"subskill" : "delivery",
"weight" : NumberInt(2),
"parser_id" : [
ObjectId("5a71d88d5ef7d41964fbec1c"),
ObjectId("5a71d88d5ef7d41964fbec1e")
]
}
]
}
I need the result return the name of skill and the number of unique parser_id.
In this case, the result should be:
[
{
"skill": "stack",
"quantity": 4
}
]
where "stack" - skill name,
and "quantity" - count of unique parser_id.
ObjectId("5a71d88d5ef7d41964fbec11")
ObjectId("5a71d88d5ef7d41964fbec1b")
ObjectId("5a71d88d5ef7d41964fbec1c")
ObjectId("5a71d88d5ef7d41964fbec1e")
Can some one help me with this request ???

Given the document supplied in your question, this command ...
db.collection.aggregate([
{ $unwind: "$connects" },
// count all occurrences
{ "$group": { "_id": {skill: "$skill", parser_id: "$connects.parser_id"}, "count": { "$sum": 1 } }},
// sum all occurrences and count distinct
{ "$group": { "_id": "$_id.skill", "quantity": { "$sum": 1 } }},
// (optional) rename the '_id' attribute to 'skill'
{ $project: { 'skill': '$_id', 'quantity': 1, _id: 0 } }
])
... will return:
{
"quantity" : 4,
"skill" : "stack"
}
The above command groups by skill and connects.parser_id and then gets a distinct count of those groups.
Your command includes the java tag so I suspect you are looking to execute the same command using the MongoDB Java driver. The code below (using MongoDB Java driver v3.x) will return the same result:
MongoClient mongoClient = ...;
MongoCollection<Document> collection = mongoClient.getDatabase("...").getCollection("...");
List<Document> documents = collection.aggregate(Arrays.asList(
Aggregates.unwind("$connects"),
new Document("$group", new Document("_id", new Document("skill", "$skill").append("parser_id", "$connects.parser_id"))
.append("count", new Document("$sum", 1))),
new Document("$group", new Document("_id", "$_id.skill").append("quantity", new Document("$sum", 1))),
new Document("$project", new Document("skill", "$_id").append("quantity", 1).append("_id", 0))
)).into(new ArrayList<>());
for (Document document : documents) {
logger.info("{}", document.toJson());
}
Note: this code deliberately uses the form new Document(<pipeline aggregator>, ...) instead of the Aggregators utilities to make it easier to see the translation between the shell command and its Java equivalent.

try $project with $reduce
$setUnion is used to keep only the distinct ids and finally $size used to get the distinct array count
db.col.aggregate(
[
{$project : {
_id : 0,
skill : 1,
quantity : {$size :{$reduce : {input : "$connects.parser_id", initialValue : [] , in : {$setUnion : ["$$value", "$$this"]}}}}
}
}
]
).pretty()
result
{ "skill" : "stack", "quantity" : 4 }

Java MongoClient - Type exceptions when using Aggregate

I am getting the following exception:
com.mongodb.CommandFailureException: { "serverUsed" : "192.168.3.25:27017" ,
"errmsg" : "exception: The argument to $size must be an Array,
but was of type: EOO" , "code" : 17124 , "ok" : 0.0}
Whenever I run this pipeline using the MongoClient driver for Java:
[{ "$match" : { "_id" : "1"}}, { "$project" : { "count" : { "$size" : "$tags"}}}]
What is causing this? The pipeline works fine in the Mongo shell?
Here is my Java code:
private void countArrayLength(String id, String arrayName) {
AggregationOptions options = AggregationOptions.builder().build();
DBObject matchFields = new BasicDBObject("_id", id);
DBObject match = new BasicDBObject("$match", matchFields);
DBObject projectCount = new BasicDBObject("$size", "$" + arrayName);
DBObject projectFields = new BasicDBObject("count", projectCount);
DBObject project = new BasicDBObject("$project", projectFields);
Cursor cursor = db.getCollection(collectionName)
.aggregate(asList(match, project), options);
}
DB Structure:
{
"_id" : "1",
"_class" : "com.mongodb.BasicDBObject",
"tags" : [
{
"tag" : "tagName"
},
{
"tag" : "tagName"
},
{
"tag" : "tagName"
},
{
"tag" : "tagName"
},
{
"tag" : "tagName"
}
]
}

Mongodb java Query Builder

I am new to java. How can I build this mongo query in java. Any help/hint will be appreciated.
db.places.find({loc : { $near :[ -122.934326171875,37.795268017578] , $maxDistance : 50 } ,$or:[{"uid":"at"},{"myList.$id" :ObjectId("4fdeaeeede2d298262bb80") } ] ,"searchTag" : { $regex : "Union", $options: "i"} } );

By using the QueryBuilder you can create the query you wanted. I have created it as follows.
QueryBuilder query = new QueryBuilder();
query.put("loc").near(-122.934326171875, 37.795268017578, 50);
query.or(
QueryBuilder.start("uid").is("at").get(),
QueryBuilder.start("myList.$id").is(new ObjectId("5024f2f577a59dd9736ddc38")).get()
);
query.put("searchTag").regex(Pattern.compile("Union",Pattern.CASE_INSENSITIVE));
When I print the query into the console it looks like :
{ "loc" : { "$near" : [ -122.934326171875 , 37.795268017578 , 50.0]} ,
"$or" : [ { "uid" : "at"} , { "myList.$id" : { "$oid" : "5024f2f577a59dd9736ddc38"}}] ,
"searchTag" : { "$regex" : "Union" , "$options" : "i"}
}
I didn't try it but hope it will work.

Java equivalent of scala query in mongodb

What is the equivalent cde in Java:
var result = collectionName.findOne()
println(result.get("name").toString)
To elaborate, This is my sample db:
{ "_id" : ObjectId("4ca039f7a5b75ab98a44b149"), "name" : "kaustubh", "country" : "india" }
{ "_id" : ObjectId("4ca03a85a12344a5e47bcc5c"), "name" : "rahul", "country" : "pakistan" }
{ "_id" : ObjectId("4ca03a9ea12344a5e47bcc5d"), "name" : "swapnil", "country" : "lanka" }
{ "_id" : ObjectId("4ca03b19a12344a5e47bcc5e"), "name" : "sagar", "country" : "nepal" }
i am running the following query on it:
query.put("country", "india");
DBCursor cursor = collection.find(query);
while (cursor.hasNext()) {
System.out.println(cursor.next());
}
that prints:
{ "_id" : { "$oid" : "4ca04b6b37a85ab92557218a"} , "name" : "kaustubh" , "country" : "india"}
as many times as the pair exists in the collection.
how do I formulate a query to get all the names, once and get the count for them.I read the docs, and didnt stumble upon any way to do it.

Try this
query.put("name", "kaustubh");
DBObject myDoc = collection.findOne(query);
System.out.println(myDoc);