hibernate-demo/src/test/java/com/ankurm/hibernatedemo/association/BagFetchTest.java

package com.ankurm.hibernatedemo.association;

import static org.assertj.core.api.Assertions.assertThat;
import static org.junit.jupiter.api.Assertions.assertThrows;

import jakarta.persistence.EntityManager;
import jakarta.persistence.EntityManagerFactory;
import java.util.List;
import org.hibernate.SessionFactory;
import org.hibernate.stat.Statistics;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

/**
 * Backs ankurm.com post 4872, docs/12-association-mappings.md chapters "MultipleBagFetchException"
 * and "The cartesian-product trap".
 */
@SpringBootTest
class BagFetchTest {

    private static final Logger DEMO = LoggerFactory.getLogger("DEMO");

    @Autowired
    private EntityManagerFactory emf;

    @BeforeEach
    void cleanTables() {
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        em.createQuery("DELETE FROM BagBookL").executeUpdate();
        em.createQuery("DELETE FROM BagAwardL").executeUpdate();
        em.createQuery("DELETE FROM BagAuthorList").executeUpdate();
        em.createQuery("DELETE FROM BagBookS").executeUpdate();
        em.createQuery("DELETE FROM BagAwardS").executeUpdate();
        em.createQuery("DELETE FROM BagAuthorSet").executeUpdate();
        em.getTransaction().commit();
        em.close();
    }

    private Statistics stats() {
        return emf.unwrap(SessionFactory.class).getStatistics();
    }

    private Long seedListAuthor(int nBooks, int nAwards) {
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        BagAuthorList author = new BagAuthorList("List Author");
        em.persist(author);
        for (int i = 0; i < nBooks; i++) {
            em.persist(new BagBookL("Book" + i, author));
        }
        for (int i = 0; i < nAwards; i++) {
            em.persist(new BagAwardL("Award" + i, author));
        }
        em.getTransaction().commit();
        Long id = author.getId();
        em.close();
        return id;
    }

    private Long seedSetAuthor(int nBooks, int nAwards) {
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        BagAuthorSet author = new BagAuthorSet("Set Author");
        em.persist(author);
        for (int i = 0; i < nBooks; i++) {
            em.persist(new BagBookS("Book" + i, author));
        }
        for (int i = 0; i < nAwards; i++) {
            em.persist(new BagAwardS("Award" + i, author));
        }
        em.getTransaction().commit();
        Long id = author.getId();
        em.close();
        return id;
    }

    @Test
    void fetchJoiningTwoListsInOneQuery_throwsMultipleBagFetchException() {
        seedListAuthor(4, 3);
        EntityManager em = emf.createEntityManager();

        IllegalArgumentException ex = assertThrows(IllegalArgumentException.class, () ->
                em.createQuery(
                        "SELECT a FROM BagAuthorList a JOIN FETCH a.books JOIN FETCH a.awards",
                        BagAuthorList.class).getResultList());

        Throwable root = ex;
        while (root.getCause() != null && root.getCause() != root) {
            root = root.getCause();
        }
        DEMO.info("MultipleBagFetchException reproduction -- wrapper class: {}", ex.getClass().getName());
        DEMO.info("MultipleBagFetchException reproduction -- root cause class: {}", root.getClass().getName());
        DEMO.info("MultipleBagFetchException reproduction -- verbatim message: {}", root.getMessage());

        assertThat(ex.getClass().getName()).as("EntityManager.createQuery(...).getResultList() wraps it as IllegalArgumentException, NOT PersistenceException")
                .isEqualTo("java.lang.IllegalArgumentException");
        assertThat(root.getClass().getName()).isEqualTo("org.hibernate.loader.MultipleBagFetchException");
        em.close();
    }

    @Test
    void fix1_useSetsInsteadOfLists_noExceptionOneQuery() {
        seedSetAuthor(4, 3);
        EntityManager em = emf.createEntityManager();
        stats().clear();

        List<BagAuthorSet> authors = em.createQuery(
                        "SELECT DISTINCT a FROM BagAuthorSet a JOIN FETCH a.books JOIN FETCH a.awards",
                        BagAuthorSet.class)
                .getResultList();

        long queries = stats().getPrepareStatementCount();
        DEMO.info("Fix #1 (Set instead of List): {} distinct authors returned, {} queries fired", authors.size(), queries);

        assertThat(authors).hasSize(1);
        assertThat(authors.get(0).getBooks()).hasSize(4);
        assertThat(authors.get(0).getAwards()).hasSize(3);
        assertThat(queries).isEqualTo(1);
        em.close();
    }

    @Test
    void fix2_twoSeparateQueries_avoidsBagFetchExceptionEntirely() {
        Long id = seedListAuthor(4, 3);
        EntityManager em = emf.createEntityManager();
        stats().clear();

        BagAuthorList author = em.createQuery(
                        "SELECT a FROM BagAuthorList a JOIN FETCH a.books WHERE a.id = :id", BagAuthorList.class)
                .setParameter("id", id)
                .getSingleResult();
        // second, separate query for the other bag -- no exception because only one JOIN FETCH per query
        author = em.createQuery(
                        "SELECT a FROM BagAuthorList a JOIN FETCH a.awards WHERE a.id = :id", BagAuthorList.class)
                .setParameter("id", id)
                .getSingleResult();

        long queries = stats().getPrepareStatementCount();
        DEMO.info("Fix #2 (two queries): {} queries fired, books={}, awards={}",
                queries, author.getBooks().size(), author.getAwards().size());

        assertThat(author.getBooks()).hasSize(4);
        assertThat(author.getAwards()).hasSize(3);
        assertThat(queries).as("one JOIN FETCH per query, run twice").isEqualTo(2);
        em.close();
    }

    @Test
    void cartesianProduct_fetchJoiningTwoAllowedSetsExplodesRowCount() {
        seedSetAuthor(4, 3);
        EntityManager em = emf.createEntityManager();

        // Raw SQL join without DISTINCT to see the true row count the database returns.
        Object rawCount = em.createNativeQuery(
                        "SELECT COUNT(*) FROM bag_author_set a " +
                                "JOIN bag_book_s b ON b.author_id = a.id " +
                                "JOIN bag_award_s w ON w.author_id = a.id")
                .getSingleResult();
        long rawRowCount = ((Number) rawCount).longValue();

        List<BagAuthorSet> entities = em.createQuery(
                        "SELECT DISTINCT a FROM BagAuthorSet a JOIN FETCH a.books JOIN FETCH a.awards",
                        BagAuthorSet.class)
                .getResultList();

        DEMO.info("Cartesian product: 4 books x 3 awards for 1 author -> raw SQL join rows = {}, distinct entities returned = {}",
                rawRowCount, entities.size());

        assertThat(rawRowCount).as("SQL returns one row per (book, award) pair: 4 x 3").isEqualTo(12);
        assertThat(entities).as("Hibernate's DISTINCT root-entity de-duplication collapses this back to 1 entity")
                .hasSize(1);
        assertThat(entities.get(0).getBooks()).hasSize(4);
        assertThat(entities.get(0).getAwards()).hasSize(3);
        em.close();
    }
}