How to perform grouping with DBRef in Spring Data MongoDB -

i need aggregation number of comments related user.

the entity "comment entity" 1 has reference of user whom commentary directed.

@document(collection = commententity.collection_name) public class commententity {      public final static string collection_name = "comments";      @id     private objectid id;      @field("message")     private string message;      @field("likes")     private long likes = 0l;      @field("created_time")     private date createdtime;      @field("target")     @dbref     private sonentity sonentity;      public commententity() {     }      @persistenceconstructor     public commententity(string message, date createdtime, sonentity sonentity) {         this.message = message;         this.createdtime = createdtime;         this.sonentity = sonentity;     }      public objectid getid() {         return id;     }      public void setid(objectid id) {         this.id = id;     }      public string getmessage() {         return message;     }      public void setmessage(string message) {         this.message = message;     }      public long getlikes() {         return likes;     }      public void setlikes(long likes) {         this.likes = likes;     }      public sonentity getsonentity() {         return sonentity;     }      public void setsonentity(sonentity sonentity) {         this.sonentity = sonentity;     }      public date getcreatedtime() {         return createdtime;     }      public void setcreatedtime(date createdtime) {         this.createdtime = createdtime;     }      @override     public string tostring() {         return "commententity{" + "id=" + id + ", message=" + message + ", likes=" + likes + ", createdtime=" + createdtime + '}';     } } 

i have implemented custom repository method following approach:

public class commentrepositoryimpl implements commentrepositorycustom {      @autowired     private mongotemplate mongotemplate;      @override     public list<commentsbysondto> getcommentsbyson() {          typedaggregation<commententity> commentsaggregation =                 aggregation.newaggregation(commententity.class,                 aggregation.group("sonentity.firstname").count().as("comments"),                 aggregation.project("sonentity.firstname").and("comments").previousoperation());          aggregationresults<commentsbysondto> results = mongotemplate.              aggregate(commentsaggregation, commentsbysondto.class);          list<commentsbysondto> commentsbysonresultslist = results.getmappedresults();          return commentsbysonresultslist;     }  } 

i did not work , not know how raise it.

the resulting dto follows:

public class commentsbysondto implements serializable {      private string firstname;     private long comments;      public commentsbysondto(){}      public commentsbysondto(string firstname, long commets) {         this.firstname = firstname;         this.comments = commets;     }      public string getfirstname() {         return firstname;     }      public void setfirstname(string firstname) {         this.firstname = firstname;     }      public long getcomments() {         return comments;     }      public void setcomments(long comments) {         this.comments = comments;     } } 

thanks in advance.