Entity class mapping in hibernate, complex relation

Question

I have four classes (A, B, C, D) all connected by relationship. I am using hibernate to store them in mysql database.

Structure of classes

Class A {
    Integer id;           //primary key
    List<B> listOfB;      //one to many with B class, able to store order of B's

    //some other fields
}

/* This class can contain list of c, or list of d, or both.
 * need to keep order among c and d also if contains both list
 * for eg, B has list of c(c1, c2, c3) and D (d1, d2, d3)
 * order can be c1, d1, c2, d3, c3, d2
 */
Class B {
    Integer id;           //primary key
    List<C> listOfC;      //one to many with D class, able to store order of C's
    List<D> listOfD;      //one to many with C class, able to store order of D's

    //How to store order between list of c and list of d?       

     //some other field
}

Class C {
    Integer id;           //primary key
    List<D> listOfD;      //one to many with D class, able to store order of D's

     //some other field
}

Class D {
    Integer id;           //primary key
    String value;         //some value

}

Here relation between A and B is one to many, B to C is one to many, B to D is one to many, C to D is one to many.

List has been used just to keep track of order between objects. But I want also to keep track of order of list of c and d in B class.

B can have following steps:
 1. C1
 2. D1
 3. c2
 4. D2

With the current design I am not able to store the order between c and d.

please suggest some design. Thank you

davioooh · Accepted Answer · 2014-05-16 15:52:19Z

2

You could inherit C and D from an abstract entity and use a single list in B.

The abstract entity:

@Entity
@Inheritance
@DiscriminatorColumn(name = "discr")
public abstract class X {
    Integer id; 
    // ...
}

Then extend it:

@Entity
@DiscriminatorValue("c")
public class C extends X {
    List<D> listOfD;
    // ...
}

@Entity
@DiscriminatorValue("d")
public class D extends X {
    String value;     
    // ...
}

and finally:

@Entity
public class B {
    Integer id;
    List<X> listOfX; // single list for C and D
}

Take a look at inheritance stategies. In my example I'm using single table strategy.

Also take a look to this

Obviously you need to add relationship annotations.

edited May 16, 2014 at 15:52

answered May 16, 2014 at 13:24

davioooh

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2 Comments

Manish Over a year ago

but how hibernate will store them in db? The structure of C and D can be different

davioooh Over a year ago

@Raj take a look at Hibernate documentation (link in the answer). You can have several tables (one for each type) or just a single table one with all the fields, plus a discriminator column.

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