class Parent {
  @Id
  private long id;

  @OneToOne(mappedBy = "parent")
  private Child child;
}

@Entity
class Child {
  @Id
  private long id;

  @OneToOne
  private Parent parent;
}

select
        parent0_.id as id1_1_ 
    from
        Parent parent0_ 
    where
        parent0_.id is null

package org.hibernate.bugs;

import static org.assertj.core.api.Assertions.assertThat;

import java.util.List;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Id;
import javax.persistence.OneToOne;
import javax.persistence.Persistence;
import javax.persistence.Query;

import org.junit.After;
import org.junit.Before;
import org.junit.Test;

/**
 * This template demonstrates how to develop a test case for Hibernate ORM, using the Java Persistence API.
 */
public class JPAUnitTestCase {

  private EntityManagerFactory entityManagerFactory;

  @Before
  public void init() {
    entityManagerFactory = Persistence.createEntityManagerFactory("templatePU");
  }

  @After
  public void destroy() {
    entityManagerFactory.close();
  }

  // Entities are auto-discovered, so just add them anywhere on class-path
  // Add your tests, using standard JUnit.
  @Test
  public void hhh123Test() throws Exception {
    EntityManager entityManager = entityManagerFactory.createEntityManager();

    runInTransaction(entityManager, () -> {
      Parent parent1 = new Parent(1);
      new Child(3, parent1);
      entityManager.persist(parent1);

      Parent parent2 = new Parent(2);
      entityManager.persist(parent2);
    });

    runInTransaction(entityManager, () -> {
      Query query = entityManager.createQuery("from Parent p where p.child is null");
      @SuppressWarnings("unchecked")
      List<Parent> parents = query.getResultList();
      assertThat(parents).hasSize(1);
      assertThat(parents).element(0).extracting(Parent::getId).isEqualTo(2);
    });

    entityManager.close();
  }

  private void runInTransaction(EntityManager entityManager, Runnable runnable) {
    entityManager.getTransaction().begin();
    runnable.run();
    entityManager.getTransaction().commit();
    entityManager.clear();
  }
}


@Entity
class Parent {
  @Id
  private long id;

  @OneToOne(mappedBy = "parent", cascade = CascadeType.ALL)
  private Child child;

  Parent() {}

  public Parent(long id) {
    this.id = id;
  }

  public long getId() {
    return id;
  }

  public Child getChild() {
    return child;
  }

  void setChild(Child child) {
    this.child = child;
  }

  @Override
  public String toString() {
    return "Parent [id=" + id + ", child=" + child + "]";
  }
}


@Entity
class Child {
  @Id
  private long id;

  @OneToOne
  private Parent parent;

  Child() {}

  public Child(long id, Parent parent) {
    this.id = id;
    setParent(parent);
  }

  public Parent getParent() {
    return parent;
  }

  public void setParent(Parent parent) {
    this.parent = parent;
    parent.setChild(this);
  }

  @Override
  public String toString() {
    return "Child [id=" + id + "]";
  }
}