How to use slice and count in the same query in MongoDB - spring

Question

I need to combine slice and count in the same query, let me explain how:
I have a collection which stores comments with his replies within in an array
{
"_id" : ObjectId("5a6b14796ede6d5169ad68a7"),
"_class" : "com.social.model.comment.FirstLevelComment",
"contentId" : "5a12996de7e84e0001b93a91",
"replies" : [
{
"_id" : ObjectId("5a6b151a6ede6d5169ad68b1"),
"date" : ISODate("2018-01-26T11:46:34.202Z"),
"text" : "Reply 1"
},
{
"_id" : ObjectId("5a6b151d6ede6d5169ad68b2"),
"date" : ISODate("2018-01-26T11:46:37.357Z"),
"text" : "Reply 2"
},
{
"_id" : ObjectId("5a6b15206ede6d5169ad68b3"),
"date" : ISODate("2018-01-26T11:46:40.170Z"),
"text" : "Reply 3"
},
{
"_id" : ObjectId("5a6b15236ede6d5169ad68b4"),
"date" : ISODate("2018-01-26T11:46:43.025Z"),
"text" : "Reply 4"
},
{
"_id" : ObjectId("5a6b15256ede6d5169ad68b5"),
"date" : ISODate("2018-01-26T11:46:45.931Z"),
"text" : "Reply 5"
}
],
"date" : ISODate("2018-01-26T11:43:53.578Z"),
"text" : "This is the comment text"
}
Every First level comment is stored in a separate document, so to retrieve all comments belong to a content, I have to make a query matching by "contentId" field.
But, I only want to retrieve the first two replies of every comment, so I have to use the $slice operator.
But I have to retrieve too, the total amount of replies that a comment has, so can I do that in the same query?
I'm using spring boot with mongo repositories, so for now my query is like this
#Query(value = "{}", fields = "{ replies : { $slice : 2 }}")
public Page<FirstLevelComment> findByContentId(String contentId, Pageable page);
But don't know how to add the number of replies to that query.
EDIT:
Added query as Alex P. said
db.comment.aggregate([
{$match:{contentId: "5a12996de7e84e0001b93a91"}},
{
$project: {
_id: 1,
_class: 1,
contentId: 1,
date: 1,
text: 1,
countSize: {$size: "$replies"},
sl: {$slice: ["$replies", 2]}
}
}])

Answer 1

If you'd directly aggregate on your mongo server you would have to do this:
db.collection.aggregate([
{
$project: {
_id: 1,
_class: 1,
contentId: 1,
date: 1,
text: 1,
countSize: {$size: "$replies"},
sl: {$slice: ["$replies", 2]}
}
}
])
When using aggregation framework in your Java application with Spring Data you can't use MongoRepository. You would have to use MongoTemplate instead.
Have a look in the documentation for more details

Answer 2

You can try below aggregation using MongoTemplate.
ProjectionOperation project = Aggregation.project().and("replies").slice(2).as("first 2 comments").and("replies").size().as("count");
SkipOperation skip = Aggregation.skip(2L);
LimitOperation limit = Aggregation.limit(5);
Aggregation aggregation = Aggregation.newAggregation(project, skip, limit);
AggregationResults<Document> result = mongoTemplate.aggregate(aggregation, collectionname, Document.class);