Is Hibernate able to query associations?

soawork22 · **Joined:** Thu Mar 12, 2009 3:41 pm **Posts:** 9

Hi there,

I’m currently working on a project using hibernate to access our database. So far I like hibernate very much.

Now I have the following problem and hope anybody can help me on this.

I have two classes which are associated with one-to-many. My object tree looks similar like this:

ProductList
|- id
|- name
|- Products (0..*)
|- id
|- name
|- deleted

The entity ProductList has a set of Products assigned. I modeled that with hibernate 3 and the read write operations work.

Now I want the ProductList only to contain Products which are not marked deleted in the database.
As you can imagine the attribute deleted of the Product entity is a flag showing whether the Product was deleted (I don’t want the entity really to be deleted from database. I just want to set the flag).

What I would need is a possibility to implement such a sort of read mechanism.

What I want to do in words.

Read ProductList by id
Read products for this ProductList (lazy or not) where product.deleted = false

Is Hibernate able to do this? Ist there a possible query to solve that issue? Or do I Have to remove the one-to-many association and make 2 selects in my business logic an assign the products to the list by hand?

Best regards
Tom

Further Information:
Hibernate v3
HSQLDB 1.8.0

mmerder · **Posted:** Fri Mar 13, 2009 7:01 am

You can use Filters to filter the products-association in your list:

Code:

@Entity
@FilterDef(name="deletedFilter")
@Filter(name="deletedFilter", condition="deleted = false")
public class Product...

and in your List:

Code:

..
@Filter(name="deletedFilter", condition="deleted = false")
private Set<Product> products;

You have to enable Filters on your session, with session.enableFilter("deletedFilter")

soawork22 · **Joined:** Thu Mar 12, 2009 3:41 pm **Posts:** 9

Ok I will hava a look into filters.
Is the filtering handled by java classes not by the database system? This could be a performance issue maybe.

littypreethkr · **Posted:** Fri Mar 13, 2009 9:54 am

@Filter requires that the filter definition be enable on every session.

But in your case the condition is something like a static criteria, so you can use the @Where(clause="deleted=false"). But note that the clause attribute should be having SQL and not HQL.

soawork22 · **Joined:** Thu Mar 12, 2009 3:41 pm **Posts:** 9

Thanks for your fast answers.

I'm currently a little bit confused.

1) I'm not sure when to use criterias and when filters?
I think if I don't want some statements turning on the filter and setting the parameter I would use a citeria.

2) Is there any possibility to create a hql statement that has the same effect? Is criteria mightier than hql?

best regards
Tom

littypreethkr · **Posted:** Mon Mar 16, 2009 4:26 am

Quote:

1) I'm not sure when to use criterias and when filters?
I think if I don't want some statements turning on the filter and setting the parameter I would use a citeria.

By criteria if you mean "The Criteria API", then you have misunderstood. I just used the word "static criteria" to define your problem.

Basically what the @Where does is that, it will append what ever is in the clause attribute to the SQL generated for fetching the collection. The advantage over @Filter is that you dont need to turn on the filter for each session; it will get appended by default. The disadvantage is that you wont be able to specify dynamic parameter values. But that wont be a problem for you coz your condition is static (DELETED=1 or something like that).

Hope now you are clear.

soawork22 · **Joined:** Thu Mar 12, 2009 3:41 pm **Posts:** 9

Thanks for your help. I found a solution.

And here we go:

1.) Criterias won't work. These seems to be the same like HQL but an programmatic api approach.
Like mentioned in another post (which I don't find anymore) the restrictions and where clauses filter the parent object (product list) not the association (products).
So as far as I see you can not solve my problem with HQL or Criterias.
I can: "filter for product lists which have products which are not deleted"
I can not: "filter the association to products to only contain not deleted products"

2.) @Where (or where attribute in association mapping like <set>) work for my problem. I can define the products set to only include not deleted products.
Disadvantages:
a) I can not decide if I want them include or not (for example admin view show the deleted - user view not)
b) I have to use sql in there

3.) Filters would work too, but you have to enabled them for each session.

I don't know if it is possible. But the possibility to define conditions for the reading of associations would be very nice.

by the way: if you try out my solution be careful with caching. The where condition or filters don't work on cached entities.

littypreethkr · **Posted:** Mon Mar 16, 2009 6:04 am

Well.. you haven't mentioned what is the solution YOU found...

soawork22 · **Joined:** Thu Mar 12, 2009 3:41 pm **Posts:** 9

Oh sorry ;-)

As I mentioned I got filters and the where condition in my <set>-xml-element working.

As my business needs are that product lists never contain deleted products I use the where in the association definition. Point 2.

But if I would use deleted flags all over my entities or if I would have to dynamically switch between show deleted / show not I would choose the filter approach.