Querying Inside Entities

[nitin pai]

Is there a way in which I can query a table inside the method of an entity.
I wanted to try out a scenario where in, if table B data has to be inserted then first check whether a column of table A has been set or not. If set then, insert into table B or else throw exception.

I know this can be done in a session bean easily, but I would like to know if this is possible inside the @PrePersist method of the Entity corresponding to table B itself.

Also, for what purpose are Entity Listeners used? Can they be useful in such cases?

[nitin pai]

umm... anyone have a clue?

[Jaikiran Pai]

Originally posted by nitin pai:

I wanted to try out a scenario where in, if table B data has to be inserted then first check whether a column of table A has been set or not. If set then, insert into table B or else throw exception.

I would like to know if this is possible inside the @PrePersist method of the Entity corresponding to table B itself.

Are those 2 entities (A and B) related to each other? If yes, then in the @PrePersist callback method of B, you could traverse to A and check the attributes of A. If they don't satisfy the requisite condition then you can throw the exception.

[nitin pai]

Thanks Jai,

Yes the two tables are related. But how to traverse back. Could you please provide me an example.

[Jaikiran Pai]

Originally posted by nitin pai:


Yes the two tables are related. But how to traverse back. Could you please provide me an example.

I was thinking along these lines:

@Entity
@Table (name="A")
public class A implements Serializable {


/**
 * Id
 */
@Id
@GeneratedValue
@Column (name="id")
private long id;
 


@OneToOne (or OneToMany)
.....
private B bObj;


public B getB() {
return this.bObj;
}

public void setB(B b) {

this.bObj = b;
}


@PrePersist
public void prePersist(A aObj) {

// get hold of B from A
B b = aObj.getB();

// get the name of B
String userName = b.getName();

// do some check on the name of B
if ( userName.equals("BadName")) {

throw new RuntimeException("Really bad name. Rejecting....");
}



}

}

 
@Entity
@Table (name="B")
public class A implements Serializable {


/**
 * Id
 */
@Id
@GeneratedValue
@Column (name="id")
private long id;
 
/**
 * Name
 */
@Column (name="name")
private String name;


public String getName() {

return this.name;
}

public void setName(String n) {

this.name = n;
}

}