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Did I miss something? newbie one-to-many question

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trans

Post subject: Did I miss something? newbie one-to-many question

Posted: Fri Jan 13, 2006 4:58 am

Newbie

Joined: Fri Jan 13, 2006 4:19 am
Posts: 4

Hi!
Trying to implement the examples from the "Hibernate in Action", making a simple bidirectional one-to-many relation: the tipical ITEM-BID problem.

In the database I have one Item with two bids, one bid amount = 10, one with amount=20.

Wants to have the Item with bid.amount = 20. But as a result list I get is the Item with the two bids.

How can I get the Item with only one bid with the amount=20?

Hibernate version:
Hibernate-Version: 3.1.

Mapping documents:
ITEM:

Code:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping>
    <class name="entities.Item" table="ITEM">
        <id name="id" column="ID">
            <generator class="native"/>
        </id>
        
        <property name="name"/>
        
       <property
           name="initialPrice"
           type="long"
           column="INITIAL_PRICE"
       />
       
       <set name="bids"
          lazy="true"
          cascade="save-update"
          inverse="true">
          <key column="ITEM_ID"/>
          <one-to-many class="entities.Bid"/>
       </set>
    </class>
</hibernate-mapping>

BID:

Code:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping>
    <class name="entities.Bid" table="BID">
        <id name="id" column="ID">
            <generator class="native"/>
        </id>
       <property
           name="amount"
           type="long"
           column="AMOUNT"
       />
    </class>
</hibernate-mapping>

Code between sessionFactory.openSession() and session.close():

Code:

List items = HibernateUtil.getSession().createCriteria(Item.class)
         .createCriteria("bids")
          .add(Expression.eq("amount", new Long("10")))
          .list();

Iterator itItems = items.iterator();
        while (itItems.hasNext()) {
            Item i = (Item)itItems.next();
            Set bids = i.getBids();
            System.out.println("bids size: " + bids.size());
        }

Name and version of the database you are using:
Oracle 8

The generated SQL (show_sql=true):

Code:

09:39:20,989 DEBUG SQL:324 - select this_.ID as ID1_, this_.name as name4_1_, this_.INITIAL_PRICE as INITIAL3_4_1_, bid1_.ID as ID0_, bid1_.AMOUNT as AMOUNT5_0_ from ITEM this_, BID bid1_ where this_.ID=bid1_.ITEM_ID and bid1_.AMOUNT=?
Hibernate: select this_.ID as ID1_, this_.name as name4_1_, this_.INITIAL_PRICE as INITIAL3_4_1_, bid1_.ID as ID0_, bid1_.AMOUNT as AMOUNT5_0_ from ITEM this_, BID bid1_ where this_.ID=bid1_.ITEM_ID and bid1_.AMOUNT=?
09:39:21,067 DEBUG SQL:324 - select bids0_.ITEM_ID as ITEM3_1_, bids0_.ID as ID1_, bids0_.ID as ID0_, bids0_.AMOUNT as AMOUNT5_0_ from BID bids0_ where bids0_.ITEM_ID=?
Hibernate: select bids0_.ITEM_ID as ITEM3_1_, bids0_.ID as ID1_, bids0_.ID as ID0_, bids0_.AMOUNT as AMOUNT5_0_ from BID bids0_ where bids0_.ITEM_ID=?
bids size: 2

Thanx for helping!

Trans

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timrobertson100

Post subject:

Posted: Fri Jan 13, 2006 8:10 am

Regular

Joined: Wed Jun 29, 2005 11:14 pm
Posts: 119
Location: København

First glance looks ok. What happens when you run the SQL in the DB

select this_.ID as ID1_, this_.name as name4_1_, this_.INITIAL_PRICE as INITIAL3_4_1_, bid1_.ID as ID0_, bid1_.AMOUNT as AMOUNT5_0_ from ITEM this_, BID bid1_ where this_.ID=bid1_.ITEM_ID and bid1_.AMOUNT=10

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timrobertson100

Post subject:

Posted: Fri Jan 13, 2006 8:16 am

Regular

Joined: Wed Jun 29, 2005 11:14 pm
Posts: 119
Location: København

Sorry - My bad.

Look closely at the code.

Code:

List items = HibernateUtil.getSession().createCriteria(Item.class)
         .createCriteria("bids")
          .add(Expression.eq("amount", new Long("10")))
          .list();

This gets all the Items which have a bid on them with amount of 10

Code:

Iterator itItems = items.iterator();
        while (itItems.hasNext()) {
            Item i = (Item)itItems.next();
            Set bids = i.getBids();
            System.out.println("bids size: " + bids.size());
        } 

This loops over those items. When you execute "i.getBids();" you can see that it is looking at all the bids for that item (see the last SQL statement). This is correct behaviour - you are asking the item to show you it's bids and it has indeed got 2 bids - it's just that when you do i.getBids(), it then goes and loads the other bids for the item.

Your logic here is actually "Give me all items that have a bid of 10" and then "Show me all bids for that item"

Does that make sense?

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trans

Post subject:

Posted: Fri Jan 13, 2006 8:35 am

Newbie

Joined: Fri Jan 13, 2006 4:19 am
Posts: 4

Ok! I got the point...
But than how can I get the Item, only with the bid amount = 10. Any time I call item.getBids() he will get me all the bids for the item.

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timrobertson100

Post subject:

Posted: Fri Jan 13, 2006 8:41 am

Regular

Joined: Wed Jun 29, 2005 11:14 pm
Posts: 119
Location: København

That's correct though.

You have one item with 2 bids. You search for your item and receive one result. You then ask it for it's bids - there are 2.

If you want to get the bid that is of value 10, then you search for the bid that has a value of 10, not search for the item that has a bid of value 10.

So I guess what you want to do is search for Bid that has a value of 10 and an item id of XYZ... although why you would ever do that I don't know

P.s. - don't forget the ratings ;o)

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trans

Post subject:

Posted: Fri Jan 13, 2006 9:08 am

Newbie

Joined: Fri Jan 13, 2006 4:19 am
Posts: 4

That means it is not possible to get the Item with the Bit amount=10? I dont need the Bit with amount=10. I need the Item with bid = 10.(Ok, I know you can have it from Bid, but what if my application works with Item entities, and my method returns a list of Item entities. I dont want to go trough all the Bid entities and get the Items.).

In SQL would be this:

Quote:

SELECT i.* FROM Item i, Bid b where i.id = b.item_id and b.amount = 10

In Java i would expect to get one Item entity with one Bid entity in the bids set. (Even if I call the item.getBids() method)

Is that not possible?

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timrobertson100

Post subject:

Posted: Fri Jan 13, 2006 9:19 am

Regular

Joined: Wed Jun 29, 2005 11:14 pm
Posts: 119
Location: København

Quote:

That means it is not possible to get the Item with the Bit amount=10?

You were already doing it!!!!

Code:

SELECT i.* FROM Item i, Bid b where i.id = b.item_id and b.amount = 10

This returns the item which has a bid of 10.

Code:

List items = HibernateUtil.getSession().createCriteria(Item.class)
         .createCriteria("bids")
          .add(Expression.eq("amount", new Long("10")))
          .list(); 

Also returns the item which has a bid of 10.

In your code though, you are then asking each item for all it's bids and thus issuing another sql which is effectively:

Code:

select * from bid where item_id="VALUE YOU GOT FROM THE ABOVE"

If you want to view the items with a bid value of 10 just do

Code:

List items = HibernateUtil.getSession().createCriteria(Item.class)
         .createCriteria("bids")
          .add(Expression.eq("amount", new Long("10")))
          .list(); 

// you have the items!!! - don't go asking for any bid info - you don't care 
Iterator iter = items.iterator();
while (iter.hasNext) {
  Item item = (Item) iter.next();
  System.out.println(item.getXYZ()); // etc
}

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trans

Post subject:

Posted: Fri Jan 13, 2006 9:36 am

Newbie

Joined: Fri Jan 13, 2006 4:19 am
Posts: 4

I got it (finally) :-)
Thank you

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