Question

I would like to create an optional ManyToOne association between two entities based on whether entity1.name equals entity2.nameOf1, but I don’t want to have to maintain this relationship in my service layer by adding, removing, fetching objets, I just want that when I get an entity2, either hibernate adds a left join entity1 e1 on e1.name = e2.nameOf1 to the select query or that it waits for a call entity2.getEntity1() to query for entity1 with a where e1.name == ? with nameOf1 as argument. I have tried so far by using @JoinFormula annotation but there is very little documentation about it. I’m using hibernate version 3.5.6 with annotations.

@Entity
public class Entity1 {
   @Id
   @GeneratedValue
   private Long id;
   @Column(unique = true)
   private String name;

   ...getter&setters...
}
@Entity
public class Entity2 {
   @Id
   @GeneratedValue
   private Long id;

   private String someProp;
   private String nameOf1;

   @ManyToOne(optional = true)
   // How to join them? this doesn't work
   // @JoinColumnsOrFormulas({ @JoinColumnOrFormula(formula = 
   //      @JoinFormula(value = "nameOf1", referencedColumnName = "name")) })
   // neither does
   // @JoinColumnsOrFormulas({ @JoinColumnOrFormula(column =
   //      @JoinColumn(name = "nameOf1", referencedColumnName = "name", 
   //          insertable = false, updatable = false, nullable = true)) })
   //@Fetch(FetchMode.JOIN)
   private Entity1 entity1;

   ...getter and setters...
}

Answer 1

I think you should just use a query to do that:

select e1 from Entity1 e1 where e1.name = :name

If you really want to go this road, a simple ManyToOne with a JoinColumn should be sufficient. You need to make it non-insertable and non-updatable, though:

@ManyToOne(optional = true)
@JoinColumn(name = "nameOf1", referencedColumnName = "name", 
            insertable = false, updatable = false)
private Entity1 entity1;