Many to One instead of One to One relationship

[Rahul Babbar]

Hi All,

I have two tables in the database which have a one to one relationship between them.
However when i generate the annotated POJOs using Netbeans IDE, a many to one is generated instead of One to One.

Here are the database script.

  
CREATE TABLE A
(
  ID  INTEGER                            NOT NULL PRIMARY KEY,
  COL1   INTEGER                            NOT NULL
);

CREATE TABLE B
(
  ID    INTEGER                          NOT NULL PRIMARY KEY,
  A_ID    INTEGER                          NOT NULL,
  COL2  VARCHAR2(20)
);

ALTER TABLE B ADD CONSTRAINT B_UK_1 UNIQUE(A_ID);
ALTER TABLE B ADD CONSTRAINT B_FK_1 FOREIGN KEY (A_ID) REFERENCES A(ID);  

Because of some complexities, i cannot make A_ID as the primary key in the table B.
So, A_ID is made the unique key and is made not null and unique and then joined with A.

Here are the Pojos that are generated.

package value;
// Generated Apr 11, 2010 8:50:13 PM by Hibernate Tools 3.2.1.GA


import java.math.BigDecimal;
import java.util.HashSet;
import java.util.Set;
import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import javax.persistence.Table;

/**
 * A generated by hbm2java
 */
@Entity
@Table(name="A"
    ,schema="TEST"
)
public class A  implements java.io.Serializable {


     private BigDecimal id;
     private BigDecimal col1;
     private Set<B> bs = new HashSet<B>(0);

    public A() {
    }

	
    public A(BigDecimal id, BigDecimal col1) {
        this.id = id;
        this.col1 = col1;
    }
    public A(BigDecimal id, BigDecimal col1, Set<B> bs) {
       this.id = id;
       this.col1 = col1;
       this.bs = bs;
    }
   
     @Id 
    
    @Column(name="ID", unique=true, nullable=false, precision=22, scale=0)
    public BigDecimal getId() {
        return this.id;
    }
    
    public void setId(BigDecimal id) {
        this.id = id;
    }
    
    @Column(name="COL1", nullable=false, precision=22, scale=0)
    public BigDecimal getCol1() {
        return this.col1;
    }
    
    public void setCol1(BigDecimal col1) {
        this.col1 = col1;
    }
@OneToMany(cascade=CascadeType.ALL, fetch=FetchType.LAZY, mappedBy="a")
    public Set<B> getBs() {
        return this.bs;
    }
    
    public void setBs(Set<B> bs) {
        this.bs = bs;
    }

}




package value;
// Generated Apr 11, 2010 8:50:13 PM by Hibernate Tools 3.2.1.GA


import java.math.BigDecimal;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.Table;
import javax.persistence.UniqueConstraint;

/**
 * B generated by hbm2java
 */
@Entity
@Table(name="B"
    ,schema="TEST"
    , uniqueConstraints = @UniqueConstraint(columnNames="A_ID") 
)
public class B  implements java.io.Serializable {


     private BigDecimal id;
     private A a;
     private String col2;

    public B() {
    }

	
    public B(BigDecimal id, A a) {
        this.id = id;
        this.a = a;
    }
    public B(BigDecimal id, A a, String col2) {
       this.id = id;
       this.a = a;
       this.col2 = col2;
    }
   
     @Id 
    
    @Column(name="ID", unique=true, nullable=false, precision=22, scale=0)
    public BigDecimal getId() {
        return this.id;
    }
    
    public void setId(BigDecimal id) {
        this.id = id;
    }
@ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name="A_ID", unique=true, nullable=false)
    public A getA() {
        return this.a;
    }
    
    public void setA(A a) {
        this.a = a;
    }
    
    @Column(name="COL2", length=20)
    public String getCol2() {
        return this.col2;
    }
    
    public void setCol2(String col2) {
        this.col2 = col2;
    }

}
  

Why is a Many To One generated instead of one to one.
I dont really need B or Set of Bs in A, so i can remove it in A.
However in B, i would have preferred One to One with A(although it is almost the same as Many to One with A_Id being unique and not null).

Can somebody help me out on that.

Thank you

[James Sutherland]

Object models and data models are very different things. Any object model from data model generator is just going to give you one of the possible objects models for the data model. It just see the constraint so guesses it is a ManyToOne. If you want it to be a OneToOne then just change the generated code. The code is just meant to get you started, normally you would want to design a proper object model based on your application requirements.

[Rahul Babbar]

Thanks James.