@Entity
public class Post implements java.io.Serializable {
...

   @OneToOne(cascade = CascadeType.ALL)
   @PrimaryKeyJoinColumn
   public Car getCar() {
      return this.car;
   }
}

@Entity
public class Car implements java.io.Serializable {

...
   @OneToOne
   @PrimaryKeyJoinColumn
   public Post getPost() {
      return this.post;
   }
}

List<Post> posts = sess.createQuery("from Post p where p.car is null").list();

select post0_.id as id0_, post0_.date as date0_, post0_.description as descript3_0_, post0_.title as title0_, post0_.url as url0_ from Post post0_ where post0_.id is null