Question

I have a class Product and a class Part, where every part can only belong to one one product. Each product has a list of its parts, but a part has no reference to its product.

@Entity
@Table (name= "products")
class Product {
  @Id
  @GeneratedValue
  @Column(name = "Id")
  int id;

  @Column(name = "Name")
  String name;

  @???
  List<Part> myParts;

parts:

@Entity
@Table (name= "parts")
class Part {
  @Id
  @GeneratedValue
  @Column(name = "Id")
  int id;

  @Column(name = "Name")
  String name;
}

In my database the table ‘products’ does not store information about its parts, but the ‘parts’ table keeps track of the products in a row ‘product_id’.

products:

| id | name |

parts:

| id | name | procuct_id |

I think it is quite normal to have this contrary approaches of the OO- and the ORM “world”, but I can’t find out how to persist my objects with Hibernate to this structure!

For the one-to-many annotation I only found examples where the part-id would have been stored in the product table.
For many-to-one it seems as if I needed a reference to the product in my parts objects, isn’t it?

I hope that I am wrong! 😉

Does anybody know if there is a way to map this without chaning my class or table structure?

( I would be very, very happy if you could explain it with annotations rather than with xml 🙂 )

Answer 1

I think you mean a one-to-many unidirectional mapping, try something as follows:

@OneToMany
@JoinColumn(name="product_id")
List<Part> myParts;

You can see an example here, @OneToMany. Check out “Example 3” in that link.

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Note: Since you have the product_id as foreign key in the parts table, it is advised to also have a Product type field in the Part class.