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ManyToOneEagerNPlusOneProblem.java
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ManyToOneEagerNPlusOneProblem.java
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package tests;
import com.querydsl.jpa.impl.JPAQuery;
import nplusone.Dog;
import nplusone.QDog;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import java.util.List;
public class ManyToOneEagerNPlusOneProblem {
public static void main(String[] args) {
run("demo-el");
run("demo-hib");
}
private static void run(String persistenceUnitName) {
EntityManagerFactory emf = Persistence.createEntityManagerFactory(persistenceUnitName);
try {
EntityManager em = emf.createEntityManager();
NPlusOne.prepareData(em);
queryForDogs(em);
queryForDogsWithJoin(em);
em.close();
} finally {
emf.close();
}
}
private static void queryForDogs(EntityManager em) {
System.out.println("\nQuery without join");
NPlusOne.clear(em);
List<Dog> dogs = new JPAQuery<>(em)
.select(QDog.dog)
.from(QDog.dog)
.fetch();
System.out.println("\nALL SELECTS HAPPENED");
System.out.println("\ndogs = " + dogs);
}
private static void queryForDogsWithJoin(EntityManager em) {
System.out.println("\nQuery with join");
NPlusOne.clear(em);
List<Dog> dogs = new JPAQuery<>(em)
.select(QDog.dog)
.from(QDog.dog)
.leftJoin(QDog.dog.owner).fetchJoin() // fetchJoin necessary
// fails on EclipseLink with: identification variable must be defined for a JOIN expression
// .leftJoin(QDog.dog.owner)
// .leftJoin(QDog.dog.owner, QDog.dog) // with alias, joins, but does not really fetch
// fetchJoin and alias, against spec, but tolerated by both Hibernate and EclipseLink
// .leftJoin(QDog.dog.owner).fetchJoin()
.fetch();
System.out.println("\nALL SELECTS HAPPENED");
System.out.println("\ndogs = " + dogs);
}
}