Question

I have 2 test classes:

Class A{
   B b;
   // some other properties
}

Class B{
   // some properties
}

every instance of A has only one instance of class B, but an instance of class B can be assigned to more than one instance of class A

like :

B b = new B();
A a1 = new A();

a1.setB(b);

A a2 = new A();
a2.setB(b);

what type of association is this?
At first I was thinking about a one-to-one unidirectional but maybe is a many-to-one? but I don’t have any collection of A objects on B.

Can someone explain me what is the correct way to implements this association (using annotation)?

Answer 1

every instance of A has only one instance of class B, but an instance
of class B can be assigned to more than one instance of class A

So , A to B is the many-to-one relationship . B to A is the one-to-many relationship.

The following shows the bi-directional mapping between A and B using annotation:

@Entity
@Table(name="tableA")
Class A{

   @ManyToOne
   @JoinColumn(name = "B_ID")
   B b;

   // some other properties

}

@Entity
@Table(name="tableB")
Class B{

   @OneToMany(mappedBy = "b")
   List Set<A> listOfA= new ArrayList<A>();  

  // some other properties
}

Important points:

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

• If no @Table is specified ,by default , it is mapped to the table with the name that is equal to the unqualified class name of the entity.

• @ManyToOne defines Class A to Class B ‘s relationship is many-to-one

• In the relational database , one-to-many 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”.

  This name of this FK column can be defined explictly by the name attribute of @JoinColumn. If @JoinColumn is not specified , then default value(s) with be used for this FK column , which concatenates with the name of “one side table”, _ (underscore), and the name of the PK in the “one side table”.

• @OneToMany defines Class B to Class A ‘s relationship is one-to-many.