Spring Data JPA Projection selected fields from the DB

Question

I was testing Spring Data 1.10.4.RELEASE, following the example in Spring Data Docs http://docs.spring.io/spring-data/jpa/docs/current/reference/html/#projections

and I noticed some issues for which I have 2 questions.

First let's suppose I have these 2 entities:

@Entity
public class Person {

  @Id @GeneratedValue
  private Long id;
  private String firstName, lastName;

  @OneToOne
  private Address address;
}

@Entity
public class Address {

  @Id @GeneratedValue
  private Long id;
  private String street, state, country;
}

• Question 1:

for the following projections:

interface PersonLimited {  

  String getFirstName(); 

  AddressLimited getAddress();
}

interface AddressLimited {  

  String getCountry(); 
}

when I run findPersonByFirstNameProjectedForLimitedData

interface PersonRepository extends CrudRepository<Person, Long> {

  @Query("select p from Person p where p.firstName = ?1")
  PersonLimited findPersonByFirstNameProjectedForLimitedData(String firstName);
}

it returns exactly what expected :

{
    firstName: 'Homer',
    address: {
        country: 'USA'
    }
}

now if I look into the generated SQL, this is what I have:

SELECT person0_.firstName      AS col_0_0_, 
       address1_.id            AS id1_13_, 
       address1_.street        AS street2_13_, 
       address1_.state         AS state3_13_, 
       address1_.country       AS country4_13_
FROM   person person0_ 
       LEFT OUTER JOIN address address1_ 
                    ON person0_.addressId = address1_.id 
WHERE  person0_.firstName = ?  

The projection for the "Person" entity is selecting only "fistName", which is 100% correct because in the PersonLimited interface I've only defined "getFirstName".

But for the "Address" entity, it selects all the fields, which is wrong because in the AddressLimited interface I've only defined "getCountry", It should only select "country".

The generated query should be something like:

SELECT person0_.firstName      AS col_0_0_, 
       address1_.country       AS country4_13_
FROM   person person0_ 
       LEFT OUTER JOIN address address1_ 
                    ON person0_.addressId = address1_.id 
WHERE  person0_.firstName = ?  

so the question is, why it is not selecting only the "country" field for the Address "entity"? why it needs to select all the fields? is it a bug in Spring?

• Question 2:

for the same projection as above,

when I run findAllPersonsProjectedForLimitedData

interface PersonRepository extends CrudRepository<Person, Long> {

  @Query("select p from Person p")
  List<PersonLimited> findAllPersonsProjectedForLimitedData();
}

it returns exactly what expected :

[
     {
        firstName: 'Homer',
        address: {
            country: 'USA'
        }
     },
     {
        firstName: 'Maggie',
        address: {
            country: 'USA'
        }
     }
]

now if I look into the generated SQL, this is what I have:

SELECT person0_.id                 AS id1_18_, 
       person0_.firstName          AS firstName2_18_, 
       person0_.lastName           AS lastName3_18_, 
       person0_.addressid          AS company4_18_
FROM   person person0_ 

SELECT address0_.id         AS id1_13_0_, 
       address0_.street     AS street2_13_0_, 
       address0_.state      AS state3_13_0_, 
       address0_.country    AS country4_13_0_
FROM   address address0_ 
WHERE  address0_.id = ? 

here, the projection for both the Person and the Address entities is selecting all the fields which is wrong, it should only select "firstName" and "country".

The generated query should be something like:

SELECT person0_.firstName        AS firstName2_18_
FROM   person person0_ 

SELECT address0_.country    AS country4_13_0_
FROM   address address0_ 
WHERE  address0_.id = ? 

is this the normal behavior, shouldn't select only the fields that we need?

Thanks,

Answer 1

If you want use the annotation @Query with Spring Data Projections you have to use field alias and you need to make sure you alias the projects matching the projection fields. The following code should work for question 1:

interface PersonRepository extends CrudRepository<Person, Long> {

  @Query("select p.firstName as firstname, p.address as address from Person p where p.firstName = ?1")
  PersonLimited findPersonByFirstNameProjectedForLimitedData(String firstName);
}

Another alternative that you can use is define your queries with Property Expressions. whenever is possible:

interface PersonRepository extends CrudRepository<Person, Long> {

  List<PersonLimited> findByFirstName(String firstName);
}