Update one of the field in documents while fetching results using criteria query from Spring Data Mongo

Question

Using criteria query I am fetching documents from MongoDB. What is my requirement here is I want update a field from parent document value by querying sub document using Criteria from Spring Data Mongo. The parent document is comments and nested documents is replies. I am able to get list of comments along with sub documents using the below code,

Data Set:
{
"_id" : ObjectId("5cb726937a7148376094d393"),
"_class" : "vzi.cpei.Comments",
"text" : "first comment on money control",
"replies" : [        
    {
        "_id" : "3cfef1cd-e0da-4883-86a4-17b223639087",
        "text" : "extract the traces",
        "status" : true
    },
    {
        "_id" : "3cfef1cd-e0da-4883-86a4-17b153690087",
        "text" : "replied deiberate",
        "status" : false
    },
    {
        "_id" : "3cfef1cd-e0da-4883-86a4-17b153139087",
        "text" : "Bgm",
        "status" : true
    }],
}
Response DTO:
      
      public class CommentsDTO{
      private String id;
      private String text;
      private List<Replies> replies;
      private Integer totalReplies;
    }

Code wrote in Spring using Spring data mongo Criteria query,

    Query query = new Query();
    Criteria criteria =Criteria.where("_id").is(new ObjectId("5efe3d1f8a2ef008249f72d9"));
    query.addCriteria(criteria);
    List<Comments> comments = mongoOps.find(query,"Comments", 
  CommentsDTO.class);
    return comments;

In the result I want to update the field repliesCount with total number of replies with status true so the expected output should be,

{
"_id" : ObjectId("5cb726937a7148376094d393"),
"_class" : "vzi.cpei.Comments",
"text" : "first comment on money control",
"totalReplies" : 2
"replies" : [        
    {
        "_id" : "3cfef1cd-e0da-4883-86a4-17b223639087",
        "text" : "extract the traces",
        "status" : true
    },
    {
        "_id" : "3cfef1cd-e0da-4883-86a4-17b153690087",
        "text" : "replied deiberate",
        "status" : false
    },
    {
        "_id" : "3cfef1cd-e0da-4883-86a4-17b153139087",
        "text" : "Bgm",
        "status" : true
    }],
}

I am totally confused where to do this operation while fetching.

Answer 1

You need to perform the MongoDB aggregation.

Explanation of pipeline

1. We apply the $match stage to filter results (similar to .find(query))
2. We apply the $project to transform the document structure and include totalReplies field calculated based on replies values

---

Shell

db.Comments.aggregate([
  {
    $match: {
      "_id": ObjectId("5cb726937a7148376094d393")
    }
  },
  {
    $project: {
      _class: 1,
      replies: 1,
      totalReplies: {
        $size: {
          "$filter": {
            input: "$replies.status",
            as: "status",
            cond: "$$status"
          }
        }
      }
    }
  }
])

MongoPlayground

Spring-Mongo

import org.springframework.data.mongodb.core.aggregation.Aggregation;
import org.springframework.data.mongodb.core.aggregation.AggregationResults;
import org.springframework.data.mongodb.core.aggregation.ArrayOperators;
import static org.springframework.data.mongodb.core.aggregation.Aggregation.*;
...

Aggregation agg = Aggregation.newAggregation(
        match(Criteria.where("_id").is(new ObjectId("5cb726937a7148376094d393"))),
        project("_id", "_class", "replies").and(ArrayOperators.Size.lengthOfArray(
                ArrayOperators.Filter.filter("replies.status").as("filter").by("$$filter")
                )).as("totalReplies"));

//System.out.println(agg);
return mongoOps.aggregate(agg, mongoOps.getCollectionName(CommentsDTO.class), CommentsDTO.class);

EDIT: Legacy Spring-boot 1.4.2

project("_id", "_class", "replies")
        .and(AggregationFunctionExpressions.SIZE
            .of(new BasicDBObject("$filter",
                new BasicDBObject("input", "$replies.status")
                    .append("as", "status")
                    .append("cond", "$$status"))))
        .as("totalReplies")