Spring data jdbc mapping not working if use not primary key - spring-boot

Question

I have 2 entities:
#Data
#Table("main_entities")
public class MainEntity {
#Id
private Long id;
private String anotherId;
#MappedCollection(idColumn = "main_entity_id")
private SecondEntity secondEntity;
}
#Data
#Table("second_entities")
public class SecondEntity {
#Id
private Long id;
private Long mainEntityId;
}
And exists the repository:
public interface MainEntityRepository extends CrudRepository<MainEntity, Long> {
#Query("SELECT * FROM main_entities WHERE another_id = :anotherId")
Optional<MainEntity> findByAnotherId(#Param("anotherId") String anotherId);
}
When I use the MainEntityRepository#findById(Long) - the SecondEntity is available, when I use the MainEntityRepository#findByAnotherId(String) - the SecondEntity is null
Update 2021.12.15:
if set the
#MappedCollection(idColumn = "main_entity_id")
private Set<SecondEntity> secondEntities;
Its allows to get the mapped collection via MainEntityRepository#findByAnotherId(String)

Answer 1

Spring Data JDBC loads 1:1 relationships with a single join and expects you to do the same when you specify a custom query.
In order to avoid ambiguities you have to use column aliases which prefix the columns with the property name of the 1:1 relation ship plus an _.
So your select should look like this:
SELECT M.ID, M.ANOTHER_ID, S.ID AS SECONDENTITY_ID, S.MAIN_ENTITY_ID AS SECONDENTITY_MAIN_ENTITY_ID
FROM MAIN_ENTITIES M
JOIN SECOND_ENTITIES S
ON M.ID = S.MAIN_ENTITY_ID
WHERE ANOTHER_ID = :anotherId
I created a complete example.
Side note: I recommend not to have an id on the non-aggregate-root entities, nor to have the reference back to the aggregate root in these entities. See Spring Data JDBC - How do I make Bidirectional Relationships?

Answer 2

so you want to fetch the second entity together with your main entity with your custom method?
I thinkt it has to do with the fetch type of your main entity. It is lazy by default and if you want to load both entitys you can try to set the fetch type to eager for the second entity field in your main entity. But be aware that this is not always the best option but rather a quick fix. See here for more information about fetch types.
You can also try using the join fetch as described in the accepted answer here to achieve your requested behaviour. I think that this would be the best solution.
I hope I got your question right if not please try to explain with further detail.