查询 JPA 规范中的连接表

Question

I want to make a request to the endpoint /users?numberOfBooksGreatherThan=5 , i want the result to be a list of users with more than 5 books each. I use the specification API to implements it.

This is what i tried:

public static Specification<User> userNumberOfBooksGreaterThan(Long number){
    return ((root, query, criteriaBuilder) -> {
        Join<User, Book> userBooks = root.join("userBooksList");
        return criteriaBuilder.greaterThan(criteriaBuilder.count(userBooks.get("owner")), number);
    });
}

But i got this error

org.postgresql.util.PSQLException: ERROR: aggregate functions are not allowed in WHERE

This is the SQL query executed behind (from logs):

select b1_0.user_id,b1_0.name from users b1_0 join books b2_0 on b1_0.user_id=b2_0.owner_id where count(b2_0.owner_id)>5

i saw some answer to a problem like this but it was just in SQL, saying that i should change where by having but i don't know how to do that with JPA Specification.

when i use the query below (left outer join + group by + having) , it returns the correct result, but i don't know how to transform this query into a jpa specification.

select b1_0.user_id,b1_0.name, from users b1_0 left outer join books b2_0 on b1_0.user_id=b2_0.owner_id group by user_id having count(b2_0.owner_id)>=5;

This is the User entity definition

@Entity(name = "users")
public class User {
    @Id
    private Long userId;
    private String name;
    @JsonManagedReference
    @OneToMany(mappedBy = "owner")
    private List<Book> userBooksList;
}

This is the Book entity definition

@Entity(name = "books")
public class Book {
    @Id
    private Long bookId;
    @ManyToOne
    @JoinColumn(name = "ownerId",referencedColumnName = "userId",nullable = false)
    private User owner;
    private Float price;
}

Thank you for your help

Answer 1

I think you have a typo (Should be greaterThan instead of greatherThan).

EDIT: You should reverse the query, instead of Join<Book,User> it should be Join<User,Book>. owner is a field of User entity, not reachable through Book.

public static Specification<User> userNumberOfBooksGreaterThan(Long number) {
return (root, query, criteriaBuilder) -> {
    Join<User, Book> userBooks = root.join("userBooksList");
    return criteriaBuilder.greaterThan(criteriaBuilder.count(userBooks), number);
};}

Changing the comparison in the criteriaBuilder will give you the correct count of books for each user.

Greetings!

Answer 2

I'm not sure the SQL statement you are trying to get:

select b1_0.user_id, b1_0.name from users b1_0 
 left outer join books b2_0 on b1_0.user_id=b2_0.owner_id 
group by user_id 
having count(b2_0.owner_id)>=5;

is syntactically correct, at least for Oracle it is not. However there are following options:

1. Having/group by

(root, cq, cb) -> {
   Join<User, Book> books = root.join("userBooksList");
   cq.having(cb.greaterThanOrEqualTo(cb.count(books), number));
   cq.groupBy(root);
   return cb.and();
}

or

(root, cq, cb) -> {
   Root<Book> bookRoot = cq.from(Book.class);
   cq.having(cb.greaterThanOrEqualTo(cb.count(bookRoot), number));
   cq.groupBy(root);
   return cb.equal(bookRoot.get("owner"), root);
}

2. correlated subquery (this one produces syntactically correct SQL statement)

(root, cq, cb) -> {
   Subquery<Long> subq = cq.subquery(Long.class);
   Root<Book> bookRoot = subq.from(Book.class);
   subq.where(cb.equal(bookRoot.get("owner"), subq.correlate(root)));
   return cb.greaterThanOrEqualTo(subq.select(cb.count(bookRoot)), number);
}