HHH-10971 - Document flush operation order

documentation/src/main/asciidoc/userguide/chapters/flushing/Flushing.adoc

@@ -234,4 +234,50 @@ The `INSERT` statement was not executed because the persistence context because
 [NOTE]
 ====
 This mode is useful when using multi-request logical transactions and only the last request should flush the persistence context.
-====
+====
+
+[[flushing-order]]
+=== Flush operation order
+
+From a database perspective, a row state can be altered using either an `INSERT`, an `UPDATE` or a `DELETE` statement.
+Because https://vladmihalcea.com/2014/07/30/a-beginners-guide-to-jpahibernate-entity-state-transitions/[entity state changes] are automatically converted to SQL statements, it's important to know which entity actions are associated to a given SQL statement.
+
+`INSERT`:: The `INSERT` statement is generated either by the `EntityInsertAction` or `EntityIdentityInsertAction`. These actions are scheduled by the `persist` operation, either explicitly or through cascading the `PersistEvent` from a parent to a child entity.
+`DELETE`:: The `DELETE` statement is generated by the `EntityDeleteAction` or `OrphanRemovalAction`.
+`UPDATE`:: The `UPDATE` statement is generated by `EntityUpdateAction` during flushing if the managed entity has been marked modified. The https://vladmihalcea.com/2014/08/21/the-anatomy-of-hibernate-dirty-checking/[dirty checking mechanism] is responsible for determining if a managed entity has been modified since it was first loaded.
+
+Hibernate does not execute the SQL statements in the order of their associated entity state operations.
+
+To visualize how this works, consider the following example:
+
+[[flushing-order-example]]
+.Flush operation order
+====
+[source, JAVA, indent=0]
+----
+include::{sourcedir}/FlushOrderTest.java[tags=flushing-order-example]
+----
+
+[source, SQL, indent=0]
+----
+include::{extrasdir}/flushing-order-example.sql[]
+----
+====
+
+Even if we removed the first entity and then persist a new one, Hibernate is going to execute the `DELETE` statement after the `INSERT`.
+
+[TIP]
+====
+The order in which SQL statements are executed is given by the `ActionQueue` and not by the order in which entity state operations have been previously defined.
+====
+
+The `ActionQueue` executes all operations in the following order:
+
+. `OrphanRemovalAction`
+. `EntityInsertAction` or `EntityIdentityInsertAction`
+. `EntityUpdateAction`
+. `CollectionRemoveAction`
+. `CollectionUpdateAction`
+. `CollectionRecreateAction`
+. `EntityDeleteAction`
+

documentation/src/main/asciidoc/userguide/chapters/flushing/extras/flushing-order-example.sql

@@ -0,0 +1,5 @@
+INSERT INTO Person (name, id)
+VALUES ('John Doe', 2L)
+
+DELETE FROM Person WHERE id = 1
+

documentation/src/test/java/org/hibernate/userguide/flush/FlushOrderTest.java

@@ -0,0 +1,89 @@
+/*
+ * Hibernate, Relational Persistence for Idiomatic Java
+ *
+ * License: GNU Lesser General Public License (LGPL), version 2.1 or later.
+ * See the lgpl.txt file in the root directory or <http://www.gnu.org/licenses/lgpl-2.1.html>.
+ */
+package org.hibernate.userguide.flush;
+
+import javax.persistence.Entity;
+import javax.persistence.Id;
+
+import org.hibernate.jpa.test.BaseEntityManagerFunctionalTestCase;
+
+import org.junit.Test;
+
+import org.jboss.logging.Logger;
+
+import static org.hibernate.userguide.util.TransactionUtil.doInJPA;
+
+/**
+ * @author Vlad Mihalcea
+ */
+public class FlushOrderTest extends BaseEntityManagerFunctionalTestCase {
+
+    private static final Logger log = Logger.getLogger( FlushOrderTest.class);
+
+    @Override
+    protected Class<?>[] getAnnotatedClasses() {
+        return new Class<?>[]{
+                Person.class
+        };
+    }
+
+    @Test
+    public void testOrder() {
+        doInJPA( this::entityManagerFactory, entityManager -> {
+            entityManager.createNativeQuery("delete from Person").executeUpdate();
+        });
+        doInJPA( this::entityManagerFactory, entityManager -> {
+            Person person = new Person("John Doe");
+            person.id = 1L;
+            entityManager.persist(person);
+        });
+        doInJPA( this::entityManagerFactory, entityManager -> {
+            log.info("testFlushSQL");
+            //tag::flushing-order-example[]
+            Person person = entityManager.find( Person.class, 1L);
+            entityManager.remove(person);
+
+            Person newPerson = new Person( );
+            newPerson.setId( 2L );
+            newPerson.setName( "John Doe" );
+            entityManager.persist( newPerson );
+            //end::flushing-order-example[]
+        });
+    }
+
+    @Entity(name = "Person")
+    public static class Person {
+
+        @Id
+        private Long id;
+
+        private String name;
+
+        public Person() {
+        }
+
+        public Person(String name) {
+            this.name = name;
+        }
+
+        public Long getId() {
+            return id;
+        }
+
+        public void setId(Long id) {
+            this.id = id;
+        }
+
+        public String getName() {
+            return name;
+        }
+
+        public void setName(String name) {
+            this.name = name;
+        }
+    }
+}