MongoDB aggregation for paginated results to Java code when grouping adds additional fields

Question

Using this answer I have created a Mongodb query that brings objects paginated and sorted and also adds the total no. of objects. What I can't do now is translate that into java Code

db.messages.aggregate([
    { $match: { _id: {$exists: true} },
    { $sort: { _id: 1 } }, // here you can sort using other field 
    { $group: {
        _id: null,
    messagesCount: { $sum: 1 },
    allMessages: {
      $push: '$$ROOT'
    }
  } }
  { $project: {
    _id: 0,
    messagesCount: 1,
    messagesPage: {
      $slice: ['$allMessages', 0, 30] //pageNo=0, pageSize=30
    }
  } }
])

MatchOperation and SortOperation were pretty straight forward. java Code:

MatchOperation matchOperation = new MatchOperation(Criteria.where("_id").exists(true));
SortOperation sortOperation = new SortOperation(new Sort(Sort.Direction.DESC, "_id"));
//HOW DO I TRANSLATE THESE TWO IN JAVA CODE?
GroupOperation groupOperation = Aggregation.group()....**???**
ProjectionOperation projectOperation = Aggregation.project()...**???**


mongoTemplate.aggregate(
            newAggregation(matchOperation, sortOperation, groupOperation, 
            projectOperation), 
            "messages", 
            MessagesSortedAndPaginated.class);

MessagesSortedAndPaginated class:

public class MessagesSortedAndPaginated {
    private long totalCount;
    private List<Message> messagesPage;
}

Message class:

 @Document(collection = "messages")
 public @Data class Message implements Serializable {

        private static final long serialVersionUID = 1L;

        @Id
        private String id;
    ...

score 0 · Answer 1 · answered Jun 21 '18 at 08:19

I've literally spent hours to fix this. Here are the missing operations:

GroupOperation groupOperation = Aggregation.group().count().as("messagesCount").push(Aggregation.ROOT).as("messagesPage");
ProjectionOperation projectOperation = Aggregation.project().andExpression("messagesCount").as("messagesPage")

score 0 · Answer 2 · answered Jun 21 '18 at 22:13

You can reach this one, using $skip and $limit in MongoDB Aggregation Pipeline, for example:

{
    "aggregate":  "messages",
    "pipeline": [
         {
             "$match": {
                 "$or":
                     [{"resourceType": "email"}, {"resourceType": "address"},{"resourceType": "telephone"} ]
             }
         },
         {
             "$project": {
                "ID":       "$resources.id",
                "CLIENTID": "$resources.clientId"
                .
                .
             }
         },{
             "$skip": ${fromId}
         }
         ,{
             "$limit": ${fetchSize}
         }
    ]
}

Also execute using MongoTemplate:

DBObject dbObject = (BasicDBObject) JSON.parse(scriptNoSql);
                if (null == dbObject) {
                    return;
                }
                DB db = mongoTemplate.getDb();
                CommandResult result = db.command(dbObject);


                if(!result.ok()) {
                    throw result.getException();
                }

MongoDB aggregation for paginated results to Java code when grouping adds additional fields

2 Answers2