Unidirectional associations with join tables(many-to-one)

heatonn1 · **Joined:** Wed Oct 24, 2012 1:27 am **Posts:** 3

Hi,

I am new for hibernate.I use unidirectional associations with join tables(many-to-one) according to Hibernate3.6 Reference Documentation below.When I query by Person,the generated sql always has a out left join on table 'PersonAddress'.Does it make sense?Is there a lazy way to only query table 'Person' when i do not use 'address' attribute in Person Object?

<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<join table="PersonAddress"
optional="true">
<key column="personId" unique="true"/>
<many-to-one name="address"
column="addressId"
not-null="true"/>
</join>
</class>
<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
</class>

s.goetz · **Posted:** Wed Oct 24, 2012 4:43 am

Is there a certain reason why you use a join table on many-to-one association? A @JoinColumn in the Address would be sufficient. If not then you actually have a many-to-many association.

To answer your question: I don't know. But I guess that hibernate needs the foreign key to initialize the proxies in your "Person.address" property.

heatonn1 · **Joined:** Wed Oct 24, 2012 1:27 am **Posts:** 3

s.goetz wrote:

Is there a certain reason why you use a join table on many-to-one association? A @JoinColumn in the Address would be sufficient. If not then you actually have a many-to-many association.

To answer your question: I don't know. But I guess that hibernate needs the foreign key to initialize the proxies in your "Person.address" property.

Hi,

Thank you for your reply.
The cause is that I can not make a decision on database schema,the tables are already designed by SA.
Do you know what the necessary of hibernate initializing the proxy is?
The table 'Person' is so basic in our system that many functions will use it.
However,the attribute 'address' is not popular,I think the SQL is not supposed to having out join always.

s.goetz · **Posted:** Wed Oct 24, 2012 8:46 am

heatonn1 wrote:

Do you know what the necessary of hibernate initializing the proxy is?

I am really no hibernate expert at all. Just started using it in may :-)
But even if an association (be it a to-one or a collection) is loaded lazily (default) the actual entity holding the association (the Person in your case) has a proxy set in place of the actual associated entities. This proxy holds the primary key of the target table so if the property of that entity is accessed later on with an open hibernate session it can be loaded with an extra query from the database or throw a LazyInitializationException if the session is closed.
As already mentioned this is why I am guessing that hibernate makes a join on your PersonAddress table. Maybe one of the experts here can correct me if I am wrong or explain more detailed.

If you do not want to load the addresses at all you could use projections in combination with a result transformer to specify the properties of your Person class that should be fetched from the database.

Code:

Criteria query = getSession().createCriteria(Person.class);

// some criterion to determine the record(s)
query.add(Restrictions.like("name", p_searchInput));

// projection list for property selection
ProjectionList projectionList = Projections.projectionList();
projectionList.add(Projections.property("name"), "name");
projectionList.add(Projections.property("persno"), "persno");
// any property of Person.class you want to load
query.setProjection(projectionList);

// result transformer to get results as Person instances
query.setResultTransformer(Transformers.aliasToBean(Person.class));

return query.list();

Regards

Sebastian

heatonn1 · **Joined:** Wed Oct 24, 2012 1:27 am **Posts:** 3

Many thanks for your explaining about proxy and solution.
But it seems that many places which were already used such load() method all need to change.It is a large effort,it not my meaning.
I will use native SQL for my independent function.