[FIXED] mongodb not returning the integer, mappingexception

Issue

Im trying to return an integer using mongo repository, the value i want to get is the id, below is the mongo query being used to extract the id field from the database as an integer

package com.example.messagestomongodb.repository;

import com.example.messagestomongodb.domain.CovidCases;
import org.springframework.data.mongodb.repository.MongoRepository;
import org.springframework.data.mongodb.repository.Query;
import org.springframework.stereotype.Repository;

@Repository
public interface CasesRepository extends MongoRepository<CovidCases, Integer> {

@Query(value = "{ '_id' : ?0 }", fields = "{ '_id' : 1}")
Integer returnIntegerFromDB(Integer id);


}

i am trying to check for duplicate records which contain the id when it already exists in the database, the reason for this is so i can DLQ the message as i am working with RabbitMQ

public boolean checkForDuplicateRecords(Integer id, CovidCases cases){
    Integer existingId = casesRepository.returnIntegerFromDB(id);

    if (cases.getId().equals(existingId)){
        log.info("existingid is {} and casesid is {}",existingId,cases.getId());
        return true;
    }
    return false;
}

I am getting the following error when i run the application:

Caused by: org.springframework.data.mapping.MappingException: Expected to read Document Document{{_id=1}} into type class java.lang.Integer but didn't find a PersistentEntity for the latter!

There seems to be an error getting the data from mongo but i am returning an integer and _id is set to an integer data type in mongo aswell

Solution

When you query with just value and fields in a JPA @Query, it would still return instances of your entity. Therefore, the return type of your method should be CovidCases. You can later, count number of documents to get the required result.

@Query(value = "{ '_id' : ?0 }", fields = "{ '_id' : 1}")
CovidCases returnIntegerFromDB(Integer id);

To get count directly using @Query, add the flag count to it:

@Query(value = "{ '_id' : ?0 }", fields = "{ '_id' : 1}", count = true)
Long returnIntegerFromDB(Integer id);

Answered By - Prashant Vishwakarma
Answer Checked By - Terry (JavaFixing Volunteer)