JPA: How can I read particular fields of an Entity? - spring

Question

I use Spring JPA ( Hibernate ) and have bunch of entities which are mapped onto tables.
When I use an entity to write I need many fields in it (see an example below). But when I read, I wanna sometimes read only particular fields like first/last name. How can I perform it using Spring data JPA ? ( because due to CrudRepository nature it returns the whole entity)
#Entity
#Table(name="PERSON")
#AttributeOverride(name = "id", column = #Column(name = "ID_PERSON"))
public class Person extends BaseEntity implements Serializable {
private static final long serialVersionUID = 1L;
#Column(name="LAST_NAME", length = 100, nullable = false)
private String lastName;
#Column(name="FIRST_NAME", length = 50, nullable = false)
private String firstName;
#Column(name="MIDDLE_NAME", length = 50)
private String middleName;
#Column(name="BIRTHDAY", nullable = false)
#Temporal(value = TemporalType.DATE)
private Date birthday;
#ManyToOne(fetch = FetchType.LAZY, optional = true)
#JoinColumn(name = "ID_SEX")
private Sex sex;
public Person() {
super();
}
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public String getMiddleName() {
return middleName;
}
public void setMiddleName(String middleName) {
this.middleName = middleName;
}
public Date getBirthday() {
return birthday;
}
public void setBirthday(Date birthday) {
this.birthday = birthday;
}
public Sex getSex() {
return sex;
}
public void setSex(Sex sex) {
this.sex = sex;
}
}

Answer 1

There are various possibilities.
With Spring Data JPA you can use projection (that's the name when you only select certain fields/columns of an entity/table).
You can return List of Object[] or a DTO or an Interface.
For example with interface it looks like this:
interface NamesOnly {
String getFirstname();
String getLastname();
}
interface PersonRepository extends Repository<Person, UUID> {
Collection<NamesOnly> findByLastname(String lastname);
}
As you can see the return value most not be of type Person.
Please check out the documentation:
https://docs.spring.io/spring-data/jpa/docs/current/reference/html/#projections

Answer 2

I was faced with a similar issue and I resorted to this:
Let's say you have your entity FooEntity related to repository FooRepository
To only get certain fields, let's say firstName and lastName using key I had to create a custom query in the FooRepository
In this manner
#Query("select new FooEntity(f.firstName, f.lastName) from FooEntity f where f.key = :key")
Optional<FooEntity> findCustomByKey(#Param("key") BigInteger key);
You also have to ensure that the FooEntity has the constructor accepting the values that you only want to be set or returned in this manner:
public FooEntity(String firstName, String lastName){
// Ensure the constructor is not called with null values
notNull(firstName, "Method called with null parameter (firstName)");
notNull(lastName, "Method called with null parameter (lastName)");
this.firstName = firstName;
this.lastName = lastName;
}
Please the full code below:
public class FooEntity implements Serializable {
#Id
#Column(name = "key")
private BigInteger key;
#Column(name = "first_name")
private String firstName;
#Column(name = "last_name")
private String lastName;
#Column(name = "birth_date")
private Date birthDate;
#Column(name = "hash")
private String hash;
public FooEntity(String firstName, String lastName){
// Ensure the constructor is not called with null values
notNull(firstName, "Method called with null parameter (firstName)");
notNull(lastName, "Method called with null parameter (lastName)");
this.firstName = firstName;
this.lastName = lastName;
}
// Getters and Setters
}
public interface FooRepository extends JpaRepository<FooEntity, BigInteger>{
#Query("select new FooEntity(f.firstName, f.lastName) from FooEntity f where f.key = :key")
Optional<FooEntity> findCustomById(#Param("key") BigInteger key); // This one only returns two set fields firstName and LastName and the rest as nulls
Optional<FooEntity> findById(BigInteger key) // This one returns all the fields
}