MongoDB Aggregtation Pipleline using Spring Boot [duplicate] - mongodb

Question

I am using spring data Mongodb in my project and refer the below classes for my query on grouping the results:
Student class:
#Document(collection = "student")
public class Student {
#Id
private String id;
private String firstName;
private String lastName;
//other fields
//getters & setters
}
StudentResults (dto):
public class StudentResults {
private String firstName;
private List<String> studentIds; //I need List<Student> here
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public List<String> getStudentIds() {
return studentIds;
}
public void setStudentIds(List<String> studentIds) {
this.studentIds = studentIds;
}
}
StudentServiceImpl class:
public class StudentServiceImpl implements StudentService {
#Autowired
private MongoTemplate mongoTemplate;
public List<StudentResults> findStudentsGroupByFirstName() {
TypedAggregation<Student> studentAggregation =
Aggregation.newAggregation(Student.class,
Aggregation.group("firstName").
addToSet("id").as("studentIds"),
Aggregation.project("studentIds").
and("firstName").previousOperation());
AggregationResults<StudentResults> results = mongoTemplate.
aggregate(studentAggregation, StudentResults.class);
List<StudentResults> studentResultsList = results.getMappedResults();
return studentResultsList;
}
}
Using the above code, I am able to retrieve the List<String> studentIds successfully, but I need to retrieve List<Student> students using Aggregation.group()? Can you help?

Answer 1

Change your TypedAggregation part to below and add students field to StudentResults
TypedAggregation<Student> studentAggregation = Aggregation.newAggregation(Student.class,
Aggregation.group("firstName").
push("$$ROOT").as("students"));
$$ROOT will push the whole document.
Update:
TypedAggregation<Student> studentAggregation = Aggregation.newAggregation(Student.class,
Aggregation.group("firstName").
push(new BasicDBObject
("_id", "$_id").append
("firstName", "$firstName").append
("lastName", "$lastName")).as("students"));