hibernate8-jpa4-demo/src/test/java/com/ankurm/agent/Demo03Test.java

package com.ankurm.agent;

import jakarta.persistence.EntityAgent;
import org.hibernate.Hibernate;
import org.junit.jupiter.api.Test;

import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.assertFalse;
import static org.junit.jupiter.api.Assertions.assertTrue;

/** Lazy associations are not loaded for you; fetch() loads them on request. */
class Demo03Test {

    @Test
    void lazyAssociationsNeedFetch() {
        try (Lab lab = new Lab(); Transcript t = new Transcript("03-lazy-associations-and-fetch")) {
            lab.seed();

            lab.sf.runInTransaction(EntityAgent.class, a -> {
                Book b = a.get(Book.class, "978-1");
                boolean init = Hibernate.isInitialized(b.publisher);
                t.line("agent.get(Book): publisher initialized? %s", init);
                assertFalse(init);

                Object viaGetter;
                try {
                    viaGetter = b.publisher.getName();
                } catch (RuntimeException e) {
                    viaGetter = e.getClass().getSimpleName();
                }
                t.line("book.publisher.getName() before fetch -> %s", viaGetter);

                lab.resetSql();
                Publisher p = a.fetch(b.publisher);
                t.line("agent.fetch(book.publisher) -> getName() = %s", p.getName());
                t.line("  same object as book.publisher? %s, initialized now? %s",
                        p == b.publisher, Hibernate.isInitialized(p));
                t.line("  reading the public field p.name instead -> %s", p.name);
                t.sql("  SQL", lab.sql());
                assertEquals("Manning", p.getName());
                assertEquals(1, lab.sql().size());
            });

            lab.sf.runInTransaction(EntityAgent.class, a -> {
                Publisher p = a.get(Publisher.class, lab.sf.callInTransaction(em ->
                        em.createQuery("select p.id from Publisher p", Long.class).getSingleResult()));
                boolean init = Hibernate.isInitialized(p.books);
                t.line("");
                t.line("agent.get(Publisher): books collection initialized? %s", init);
                assertFalse(init);

                lab.resetSql();
                var books = a.fetch(p.books);
                t.line("agent.fetch(publisher.books) -> %d books", books.size());
                t.sql("  SQL", lab.sql());
                assertEquals(3, books.size());
                assertTrue(Hibernate.isInitialized(books));
            });
        }
    }
}