Question

Say I have this class:

@Entity
@Table(name="PICTURE")
public class Picture{
   private String category1, category2;
}

but the database structure looks like this:

TABLE PICTURE {
   int category1;
   int category2;
   ...
}

TABLE PICTURE_REF {
   int category;
   String categoryName;
   ...
}

How would I use JPA annotations on Picture so that any time I request an instance of it, category1 and category2 contains the categoryName from the PICTURE_REF table instead of the actual integer id stored in the PICTURE table?

I’m also wondering how saves would work because the user would select a category from a dropdown and the corresponding category integer ID would be what’s stored in the PICTURE table.

Answer 1

From your description , PICTURE.category1 and PICTURE.category2 have the many-to-one relationship to the PICTURE_REF

The following shows the bi-directional mapping between them using annotation:

For table PICTURE:

@Entity
@Table(name="PICTURE")
public Class Picture{

   @Id
   private Integer id;

   @ManyToOne(fetch = FetchType.EAGER)
   @JoinColumn(name = "category1")
   private PictureRef category1, 

   @ManyToOne(fetch = FetchType.EAGER)
   @JoinColumn(name = "category2")
   private PictureRef category2;


   //getter and setters 

}

For table PICTURE_REF:

@Entity
@Table(name="PICTURE_REF")
public Class PictureRef{

   @Id
   private Integer id;

   @OneToMany(mappedBy = "category1")
   List <Picture> listOfCat1Picture= new ArrayList<Picture>();  

   @OneToMany(mappedBy = "category2")
   List <Picture> listOfCat2Picture= new ArrayList<Picture>(); 

   //getter and setters 

}

Important Points:

• @Entity marks the java class as an hibernate entity. It is mapped to the name of the table specified in the @Table

• Use @ManyToOne to define the many-to-one relationship

  In the relational database , many-to-one relationship is expressed by using the following foreign key constraint:
  “Many side table” has a FK column which only accepts the PK of the “one side table”.

  In your case , these FK columns are PICTURE.category1 and PICTURE.category2. The name of these FK columns can be explicitly defined by the name attribute of @JoinColumn.

• FetchType.EAGER makes that PictureRef will be eagerly fetched whenever Picture is loaded or get

• Depending on your requirement , you can do the unidirectional mapping by omitting @OneToMany in the PictureRef.It will also work .But given PictureRef , you cannot access its Picture

• Given a Picture instance , you can get its categoryName and categoryId by

  • picture.getCategory1().getCategoryName()
  • picture.getCategory1().getId()
  • picture.getCategory2().getCategoryName()
  • picture.getCategory2().getId()