Design issues - lazy/eager mapping

Sharon. · **Posted:** Mon Feb 18, 2008 2:08 pm

Hey,

Let’s say I have an ‘Hotel’ with one to one relation to ‘Room’ and ‘Room’ has one to one relation to ‘RoomDetails’.

When I ask for hotel - the roomDetails are useless, so the relation between room to roomDetails should be lazy.
Yet, when I ask for room, the roomDetails are needed, so the relation should be eager.

This example is just a simple one - in my application I have lot of references that refer to more references and etc.
I’ve examined the query hibernate execute when it tries to load object AAA and found that it has 31 left outer joins, most of them are irrelevant when loading AAA , yet I cant map them as lazy since they’re needed when loading BBB or CCC that are referenced from AAAA.

Any advice will be welcome

farzad · **Posted:** Mon Feb 18, 2008 2:16 pm

You could use HQL or Criteria objects to fetch a network of objects. With either one, you have the option to force loading lazy relations or leave it for a later time. This also applies to eagerly fetched relations.

Fatzad-

Sharon. · **Posted:** Wed Feb 20, 2008 9:36 am

I know about the fetch strategy.
My facade is generic and my object graph is not as simple like as in the example :)
so i don’t have special queries i just run entitymanager.find
I don’t want to write for each entity in my application special loading.

ColinEberhardt · **Joined:** Wed Feb 20, 2008 12:16 pm **Posts:** 1

Hi,

I have another alternative which is worth exploring.

Taking your example where you have a Hotel which contains Rooms which i turn contains RoomDetails. When you retrieve Hotels you want them to include Rooms but not Room Details. When you retrieve Rooms you want them to include RoomDetails.

As you are aware, you cannot specify this with the fetch attribute for each relationship.

What I do in this case is create a RefRoom, or RoomSummary entity which maps to the same table as Room but only includes the information required when I am retrieving Hotels. i.e. in this case it would not map the RoomDetails relationship, and in practice may not include a number of other attributes of the Room entity.

In this way you are then free to specify EAGER fetch for your relationships without worrying about it cascading out of control. It also means that you can use Session.get() to retrieve Hotel by its primary key without having to create HQL or queries with explicit fetches each time.

Hope this helps.

Regards,
Colin E.

hispeedsurfer · **Joined:** Wed May 09, 2007 7:06 am **Posts:** 12

Hi ColinEberhardt ,

I have exactly the same problem.

Can you give me an example how to implement this:

Code:

What I do in this case is create a RefRoom, or RoomSummary entity which maps to the same table as Room but only includes the information required when I am retrieving Hotels

I hava the following

Code:

@Entity
@Name("shortQueryResult")
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name="QUERYTYPE", discriminatorType=DiscriminatorType.STRING)
public class ShortQueryResult extends StandardEntity implements Serializable, Comparable<ShortQueryResult> {
   
   
   protected BaseCategory query;
   public void setQuery(BaseCategory query) {
      this.anfrage = query;
   }

and

Code:

@Entity(name="ShortQueryResultRef")
@Name("shortQueryResultRef")
public class ShortQueryResultRef extends ShortQueryResult {

   @OneToOne(cascade={CascadeType.REMOVE, CascadeType.ALL}, fetch=FetchType.EAGER)
   public BaseCategory getQuery() {
      return query;
   }
   
}

In my case I don't need the field query at all times. When I leave the field complete in ShortQueryResult class with no Inheritance and LAZY loading some fields in query are not availeble.

I hope you can give me an example.
Thanks
Andi