Question

Im very new to Hibernate so this will probably a easy task for you guys.

As the Topic says I’m trying to reference the same entity in multiple Lists. But when I try to do so I get an exception saying: “Duplicate entry ‘5’ for key ‘military_id'”.
I googled but could not find a solution to my problem.

I have an Entity called MilitaryUnitData like this:

@Entity
public class MilitaryUnitData implements IMovable{
   private long Id;

//snip

   @Id
   @GeneratedValue(strategy=GenerationType.TABLE)
   public long getId() {
      return Id;
   }
   public void setId(long id) {
      Id = id;
   }

       //snip
}

and a class City where I want to store my units in.

@Entity
public class CityData {

private Collection<MilitaryUnitData> military = new ArrayList<MilitaryUnitData>();
private String name;

//snip

@Id
   public String getName() {
      return name;
   }

   public void setName(String name) {
      this.name = name;
   }

@OneToMany
   @Column(nullable=false)
   public Collection<MilitaryUnitData> getMilitary() {
      return military;
   }

   public void setMilitary(Collection<MilitaryUnitData> military) {
      this.military = military;
   }

//snip
}

The problem occurs when I want to put a Unit into 2 cities at the same time.
How do I have to change the mapping to achive this?

Thx in advance.

Answer 1

I’m trying to reference the same entity in multiple Lists
After looking at your code, I think you mean, that the same MilitaryUnitData is used in several CityData?

IF this is correct, than the realtion ship is a M:N relation ship, and you need to use a @ManyToMany instead of an @OneToMany.