CODEWITHSUNDEEP
mongodbmongodb-queryaggregatecounting
Naman
Naman

Reputation: 31968

Counting the total number of relations and mapping it to a Result

Given a Post type document as:

@Getter
@AllArgsConstructor
class Post {
    String publisher;
    Set<String> interests;
}

From a collection of posts I am seeking to find out the Set<Result> such that the Result object would look like:

@Getter
@AllArgsConstructor
class Result {
    String interestId;
    String publisherId;
    Long count;
}

To compare the operation with a implementation, I am looking for something down the lines of the following method:

Set<Result> buildQuery(List<Post> postList) {
    return postList.stream()
            .flatMap(post -> post.getInterests().stream()
                    .map(interest -> new AbstractMap.SimpleEntry<>(interest, post.getPublisher())))
            .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()))
            .entrySet().stream()
            .map(e -> new Result(e.getKey().getKey(), e.getKey().getValue(), e.getValue()))
            .collect(Collectors.toSet());
}

While translating this to the mongodb query, this what I have until now:

db.posts.aggregate([
 {"$unwind" :"$interestIds"},
 {"$group" : {"_id": {interestId: "$interestIds", publisherId:"$publisherId"}, "count": {"$sum" : 1}}},
]);

This results in documents such as:

{
    "_id" : {
        "interestId" : "INTEREST1",
        "publisherId" : "PUBLISHER2"
    },
    "count" : 3.0
}

Now, I am struggling to solve for the end map operation to formulate this into an expected result which would be:

{
    "_id" : ...
    "interestId" : "INTEREST1",
    "publisherId" : "PUBLISHER2"
    "count" : 3.0
}

How can I fix my query to get the expected result?

Upvotes: 0

Views: 36

Answers (1)

mickl
mickl

Reputation: 49975

$project is an equivalent of Java's map:

db.posts.aggregate([
    {"$unwind" :"$interestIds"},
    {"$group" : {"_id": {interestId: "$interestIds", publisherId:"$publisherId"}, "count": {"$sum" : 1}}},
    {"$project":{ _id: 0, interestId: "$_id.interestId", publisherId: "$_id.publisherId", count: 1 }}
]);

Upvotes: 2

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