Hibernate 8 + Jakarta Persistence 4: EntityAgent and stateless sessions demo

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_017ic4b7jUQrypvpE1ix1k7V

src/test/java/com/ankurm/agent/Demo03Test.java

@@ -0,0 +1,61 @@
+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));
+            });
+        }
+    }
+}