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Retrieve ONLY required fields/columns using JPA

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hb_dev

Post subject: Retrieve ONLY required fields/columns using JPA

Posted: Mon Dec 24, 2007 12:05 am

Newbie

Joined: Mon Oct 15, 2007 11:44 am
Posts: 7

Hello,

I'm using JPA with Hibernate. I have a Entity 'Customer' with 30 fields which is mapped to table 'customer_table'. I want all rows but only 2 columns.
In SQL, I would have written a query:

Code:

SELECT customer_id,customer_name from customer_table

In JPA, I wrote:

Code:

Query q = em.createQuery("SELECT c.customerId, c.customerName from Customer c");
List<Customer> customerList = q.getResultList();
for(Customer customer: customerList ){
        System.out.println("customer Id=" + customer.getCustomerId() );
}

But when I try to run, I get 'java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to entity.Customer'.

Please tell how to resolve this issue.

Thank You.

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who_am_i

Post subject: Attributes not the object itself

Posted: Wed Dec 26, 2007 9:42 am

Newbie

Joined: Wed Dec 26, 2007 9:36 am
Posts: 7

You are trying to retrieve two attributes (customer_id,customer_name) and not a customer object itself. So you would have to do something like:

Code:

List results = q.getResultList();
Iterator it = results.iterator();
while (it.hasNext()) {
    Object[] result = (Object[]) it.next();
    System.out.println("customer Id=" + result[0]);
    System.out.println("customer Name=" + result[1]);
    // do whatever you want to do with these attributes e.g. crate a new customer object
}

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hb_dev

Post subject:

Posted: Thu Dec 27, 2007 11:52 am

Newbie

Joined: Mon Oct 15, 2007 11:44 am
Posts: 7

Thank You ! (credit to you !!)
Please also help here: http://forum.hibernate.org/viewtopic.php?p=2372253

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willem

Post subject: Creste a value object

Posted: Mon Jan 07, 2008 10:46 am

Newbie

Joined: Wed Aug 23, 2006 7:59 am
Posts: 7

Hi,

I don't know whether its hibernate specific or JPA, but you can create value objects using new operator;

Code:

SELECTnew CustomerListEntry(c.customerId, c.customerName) from Customer c

with CustomerListEntry having a constructor like this:

Code:

public CustomerListEntry(Long id, String name) {
...
}

This way you don't have to make the jvm create a whole customer object when all you use are the code and name attributes, lowering you memory footprint. I assume you need it for an overview page or someting?

greets,

Willem

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