HHH-11453 - Documentation: explain/state find() and Query on a single entity can behave differently

Author: vladmihalcea

documentation/src/main/asciidoc/userguide/chapters/fetching/Fetching.adoc

@@ -45,6 +45,67 @@ _dynamic_ (sometimes referred to as runtime)::
     HQL/JPQL::: and both Hibernate and JPA Criteria queries have the ability to specify fetching, specific to said query.
     entity graphs::: Starting in Hibernate 4.2 (JPA 2.1) this is also an option.
 
+[[fetching-direct-vs-query]]
+=== Direct fetching vs entity queries
+
+To see the difference between direct fetching and entity queries in regard to eagerly fetched associations, consider the following entities:
+
+[[fetching-direct-vs-query-domain-model-example]]
+.Domain model
+====
+[source, JAVA, indent=0]
+----
+include::{sourcedir}/DirectVsQueryFetchingTest.java[tags=fetching-direct-vs-query-domain-model-example]
+----
+====
+
+The `Employee` entity has a `@ManyToOne` association to a `Department` which is fetched eagerly.
+
+When issuing a direct entity fetch, Hibernate executed the following SQL query:
+
+[[fetching-direct-vs-query-direct-fetching-example]]
+.Direct fetching example
+====
+[source, JAVA, indent=0]
+----
+include::{sourcedir}/DirectVsQueryFetchingTest.java[tags=fetching-direct-vs-query-direct-fetching-example]
+----
+
+[source, SQL, indent=0]
+----
+include::{extrasdir}/fetching-direct-vs-query-direct-fetching-example.sql[]
+----
+====
+
+The `LEFT JOIN` clause is added to the generated SQL query because this association is required to be fetched eagerly.
+
+On the other hand, if you are using an entity query that does not contain a `JOIN FETCH` directive to the `Department` association:
+
+[[fetching-direct-vs-query-entity-query-example]]
+.Entity query fetching example
+====
+[source, JAVA, indent=0]
+----
+include::{sourcedir}/DirectVsQueryFetchingTest.java[tags=fetching-direct-vs-query-entity-query-example]
+----
+
+[source, SQL, indent=0]
+----
+include::{extrasdir}/fetching-direct-vs-query-entity-query-example.sql[]
+----
+====
+
+Hibernate uses a secondary select instead. This is because the entity query fetch policy cannot be overridden,
+so Hibernate requires a secondary select to ensure that the EAGER association is fetched prior to returning the result to the user.
+
+[IMPORTANT]
+====
+If you forget to JOIN FETCH all EAGER associations, Hibernate is going to issue a secondary select for each and every one of those
+which, in turn, can lean to N+1 query issues.
+
+For this reason, you should prefer LAZY associations.
+====
+
 [[fetching-strategies]]
 === Applying fetch strategies
 
@@ -82,7 +143,7 @@ include::{sourcedir}/FetchingTest.java[tags=fetching-strategies-no-fetching-exam
 ====
 
 In this example, the application gets the `Employee` data.
-However, because all associations from `Employee `are declared as LAZY (JPA defines the default for collections as LAZY) no other data is fetched.
+However, because all associations from `Employee` are declared as LAZY (JPA defines the default for collections as LAZY) no other data is fetched.
 
 If the login process does not need access to the `Employee` information specifically, another fetching optimization here would be to limit the width of the query results.
 
@@ -99,7 +160,7 @@ include::{sourcedir}/FetchingTest.java[tags=fetching-strategies-no-fetching-scal
 === Dynamic fetching via queries
 
 For the second use case, consider a screen displaying the `Projects` for an `Employee`.
-Certainly access to the `Employee `is needed, as is the collection of `Projects` for that Employee. Information about `Departments`, other `Employees` or other `Projects` is not needed.
+Certainly access to the `Employee` is needed, as is the collection of `Projects` for that Employee. Information about `Departments`, other `Employees` or other `Projects` is not needed.
 
 [[fetching-strategies-dynamic-fetching-jpql-example]]
 .Dynamic JPQL fetching example
@@ -119,7 +180,7 @@ include::{sourcedir}/FetchingTest.java[tags=fetching-strategies-dynamic-fetching
 ----
 ====
 
-In this example we have an `Employee `and their `Projects` loaded in a single query shown both as an HQL query and a JPA Criteria query.
+In this example we have an `Employee` and their `Projects` loaded in a single query shown both as an HQL query and a JPA Criteria query.
 In both cases, this resolves to exactly one database query to get all that information.
 
 [[fetching-strategies-dynamic-fetching-entity-graph]]

documentation/src/main/asciidoc/userguide/chapters/fetching/extras/fetching-direct-vs-query-direct-fetching-example.sql

@@ -0,0 +1,12 @@
+select
+    e.id as id1_1_0_,
+    e.department_id as departme3_1_0_,
+    e.username as username2_1_0_,
+    d.id as id1_0_1_ 
+from
+    Employee e 
+left outer join
+    Department d 
+        on e.department_id=d.id 
+where
+    e.id = 1

documentation/src/main/asciidoc/userguide/chapters/fetching/extras/fetching-direct-vs-query-entity-query-example.sql

@@ -0,0 +1,15 @@
+select
+    e.id as id1_1_,
+    e.department_id as departme3_1_,
+    e.username as username2_1_ 
+from
+    Employee e 
+where
+    e.id = 1
+
+select
+    d.id as id1_0_0_
+from
+    Department d
+where
+    d.id = 1

documentation/src/test/java/org/hibernate/userguide/fetching/DirectVsQueryFetchingTest.java

@@ -0,0 +1,100 @@
+/*
+ * 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.fetching;
+
+import javax.persistence.Entity;
+import javax.persistence.FetchType;
+import javax.persistence.Id;
+import javax.persistence.ManyToOne;
+
+import org.hibernate.annotations.NaturalId;
+import org.hibernate.dialect.H2Dialect;
+import org.hibernate.jpa.test.BaseEntityManagerFunctionalTestCase;
+
+import org.hibernate.testing.RequiresDialect;
+import org.junit.Test;
+
+import org.jboss.logging.Logger;
+
+import static org.hibernate.testing.transaction.TransactionUtil.doInJPA;
+
+/**
+ * @author Vlad Mihalcea
+ */
+@RequiresDialect(H2Dialect.class)
+public class DirectVsQueryFetchingTest extends BaseEntityManagerFunctionalTestCase {
+
+	private static final Logger log = Logger.getLogger( DirectVsQueryFetchingTest.class );
+
+	@Override
+	protected Class<?>[] getAnnotatedClasses() {
+		return new Class<?>[] {
+			Department.class,
+			Employee.class,
+		};
+	}
+
+	@Test
+	public void test() {
+		doInJPA( this::entityManagerFactory, entityManager -> {
+			Department department = new Department();
+			department.id = 1L;
+			entityManager.persist( department );
+
+			Employee employee1 = new Employee();
+			employee1.id = 1L;
+			employee1.username = "user1";
+			employee1.department = department;
+			entityManager.persist( employee1 );
+
+		} );
+
+		doInJPA( this::entityManagerFactory, entityManager -> {
+			//tag::fetching-direct-vs-query-direct-fetching-example[]
+			Employee employee = entityManager.find( Employee.class, 1L );
+			//end::fetching-direct-vs-query-direct-fetching-example[]
+		} );
+
+		doInJPA( this::entityManagerFactory, entityManager -> {
+			//tag::fetching-direct-vs-query-entity-query-example[]
+			Employee employee = entityManager.createQuery(
+					"select e " +
+					"from Employee e " +
+					"where e.id = :id", Employee.class)
+			.setParameter( "id", 1L )
+			.getSingleResult();
+			//end::fetching-direct-vs-query-entity-query-example[]
+		} );
+	}
+
+	//tag::fetching-direct-vs-query-domain-model-example[]
+	@Entity(name = "Department")
+	public static class Department {
+
+		@Id
+		private Long id;
+
+		//Getters and setters omitted for brevity
+	}
+
+	//tag::fetching-direct-vs-query-domain-model-example[]
+	@Entity(name = "Employee")
+	public static class Employee {
+
+		@Id
+		private Long id;
+
+		@NaturalId
+		private String username;
+
+		@ManyToOne(fetch = FetchType.EAGER)
+		private Department department;
+
+		//Getters and setters omitted for brevity
+	}
+	//end::fetching-direct-vs-query-domain-model-example[]
+}