如何使用spring CrudRepository仅选择特定字段？

Question

How can I select only specific fields from the following class hierarchy?

@Entity
public class MyEntity {
   @Id private Long id;

   @ManyToOne  
   @JoinColumn(name="fk_person_id", foreignKey = @ForeignKey(name="fk_person"))
   private Person person; //unidirectional

   private String fieldA, fieldB, ..field M;

   //many more fields and some clobs
}

@Entity
public class Person { 
   @Id private Long id;
   private String firstname;
   private String lastname;
}


interface MyEntityRepository extends CrudRepository<MyEntity, Long> {
    List<MyEntity> findByIdAndPersonFirstnameAndPersonLastname(long id, String firstname, String lastname);
}

This works perfectly, just the performance is very poor as MyEntity and also Person have some fields and associations that I would like to prevent to be fetched in this specific case (eg clob/texts).

Question: how can I write this query to find the result set, and just fetch the fields that are absolutely required (let's assume id, fieldA, fieldB from MyEntity ?

Answer 1

Use lazy initialization for the non necessary fields:

• FetchType.LAZY = Doesn't load the relationships unless explicitly “asked for” via getter
• FetchType.EAGER = Loads ALL relationships

For example, in Person :

@ManyToOne(fetch=FetchType.LAZY)
@JoinColumn(name="fk_person_id", foreignKey = @ForeignKey(name="fk_person"))
private Person person; //unidirectional

Answer 2

Mark the unwanted fields as lazy (beware of this documented warning though), or create a dedicated class with the properties you want, and use projections in your query:

select new com.foo.bar.MyEntityWithFirstNameAndLastName(m.id, person.firstname, person.lastname) 
from MyEntity m 
join m.person person
where ...

If Person is an often-used entity, and contains large blobs that should rarely be fetched, you should consider storing the blobs in a separate entity, and use a OneToOne lazy association.