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

package com.ankurm.hibernatedemo.association;

import static org.assertj.core.api.Assertions.assertThat;

import jakarta.persistence.EntityManager;
import jakarta.persistence.EntityManagerFactory;
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 chapter "The @OneToOne lazy trap".
 *
 * <p>{@link LazyUser#getProfile()} is the non-owning ("mappedBy") side of an optional
 * {@code @OneToOne}, declared {@code FetchType.LAZY}. Without bytecode enhancement, Hibernate
 * cannot build a lazy proxy for it (it has no FK column to defer against), so it fires an
 * eager extra SELECT regardless of the annotation. {@link MiUser}/{@link MiProfile} show the
 * {@code @MapsId} fix: don't map the inverse side at all, look the child up on demand by the
 * shared primary key.
 */
@SpringBootTest
class OneToOneLazyTest {

    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 LazyProfile").executeUpdate();
        em.createQuery("DELETE FROM LazyUser").executeUpdate();
        em.createQuery("DELETE FROM MiProfile").executeUpdate();
        em.createQuery("DELETE FROM MiUser").executeUpdate();
        em.getTransaction().commit();
        em.close();
    }

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

    @Test
    void mappedBySideOneToOne_firesExtraSelectEvenThoughItIsDeclaredLazy() {
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        LazyUser user = new LazyUser("alice");
        em.persist(user);
        LazyProfile profile = new LazyProfile("bio text", user);
        user.setProfile(profile);
        em.persist(profile);
        em.getTransaction().commit();
        Long id = user.getId();
        em.close();

        EntityManager em2 = emf.createEntityManager();
        stats().clear();
        LazyUser loaded = em2.find(LazyUser.class, id);
        long queriesAfterFind = stats().getPrepareStatementCount();
        DEMO.info("LazyUser.find(): {} queries fired BEFORE touching getProfile() at all (expected 2: user + eager profile join/select)", queriesAfterFind);

        // Note: the extra SELECT for profile already happened during find(), not here.
        String bio = loaded.getProfile() == null ? null : "loaded";
        long queriesAfterAccess = stats().getPrepareStatementCount();
        DEMO.info("after touching getProfile(): {} queries total (profile={})", queriesAfterAccess, bio);

        assertThat(queriesAfterFind)
                .as("mappedBy @OneToOne(LAZY) still fires the profile SELECT immediately during find(), not on first access")
                .isEqualTo(2);
        assertThat(queriesAfterAccess).as("no further query needed -- it already ran eagerly").isEqualTo(2);
        em2.close();
    }

    @Test
    void mapsIdFix_loadingUserAloneFiresOnlyOneQuery_profileFetchedOnDemand() {
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        MiUser user = new MiUser("bob");
        em.persist(user);
        em.flush(); // need the generated id before building the shared-PK child
        MiProfile profile = new MiProfile("bob's bio", user);
        em.persist(profile);
        em.getTransaction().commit();
        Long id = user.getId();
        em.close();

        EntityManager em2 = emf.createEntityManager();
        stats().clear();
        MiUser loaded = em2.find(MiUser.class, id);
        long queriesForUserOnly = stats().getPrepareStatementCount();
        DEMO.info("MiUser.find() (no mappedBy field at all): {} query", queriesForUserOnly);

        // Fetch profile ONLY when actually needed, using the shared PK -- no proxy required.
        MiProfile profileOnDemand = em2.find(MiProfile.class, id);
        long queriesAfterExplicitProfileFetch = stats().getPrepareStatementCount();
        DEMO.info("explicit MiProfile.find() by shared PK when actually needed: {} total queries", queriesAfterExplicitProfileFetch);

        assertThat(loaded).isNotNull();
        assertThat(queriesForUserOnly)
                .as("with @MapsId and no inverse mappedBy field, loading the user alone costs exactly 1 query")
                .isEqualTo(1);
        assertThat(profileOnDemand.getId()).isEqualTo(id);
        assertThat(queriesAfterExplicitProfileFetch)
                .as("fetching the profile only happens when the code actually asks for it")
                .isEqualTo(2);
        em2.close();
    }
}