JPQL无法正常工作

Question

Parent class:

@Table(name = "users")
class User {
@OneToMany(mappedBy = "user", cascade = CascadeType.ALL, fetch = FetchType.EAGER)
List<Gender> genders = new ArrayList<>();

Child class:

@Table(name = "gender")
class Gender {
@ManyToOne(cascade = {CascadeType.ALL})
@JoinColumn(name = "userId", nullable = false)
User user;
String sex;

my goal in MySQL:

SELECT * FROM users a LEFT JOIN genders b ON a.id=b.id WHERE b.sex='dasd'

Returns 1 ROW It works good. I got only one record that meets this condition.

but same code in JPQL:

SELECT a FROM Users a LEFT JOIN a.genders b WHERE b.sex ='dasd'

Returns: one user – correctly and ALL genders in table Gender, but I dont want to ALL I want only when sex ='dasd' and only one record that meets this condition.btw JPQL generate N+1 issue and for every subquery ignoring condtion sex ='dasd'

How to write subquery satisfying the condition? Can I force subquery to return only Gender matching the JPQL query?

Answer 1

In JPQL you select from entity not table,

SELECT a FROM User a LEFT JOIN a.genders b WHERE b.sex ='dasd'

Since you configured the a.genders list as EAGER, every time you select an User, it will fetch all genders the user is asociated to. Even if they don't match the JPQL query.

If you setup the a.genders list as LAZY, it wont fetch any Gender, since you selected only from User

That query is equivalent to SQL

SELECT a.* FROM users a LEFT JOIN genders b ON a.id=b.id WHERE b.sex='dasd'

If you want to select only one Gender

SELECT b FROM Gender b WHERE b.user = :user AND b.sex = 'dasd'