Constrained="true|false" what does this mean?

amishera2007 · **Joined:** Tue Jul 24, 2007 2:40 am **Posts:** 6

Hi,
I was reading the reference document of hibernate and stuck at this.

Code:

<one-to-one
        name="propertyName"                                          (1)
        class="ClassName"                                            (2)
        cascade="cascade_style"                                      (3)
        constrained="true|false"                                     (4)
        fetch="join|select"                                          (5)
        property-ref="propertyNameFromAssociatedClass"               (6)
        access="field|property|ClassName"                            (7)
        formula="any SQL expression"                                 (8)
        lazy="proxy|no-proxy|false"                                  (9)
        entity-name="EntityName"                                     (10)
        node="element-name|@attribute-name|element/@attribute|."
        embed-xml="true|false"
        foreign-key="foreign_key_name"
/>

Code:

constrained (optional) specifies that a foreign key constraint on the primary key of the mapped table references the table of the associated class. This option affects the order in which save() and delete() are cascaded, and determines whether the association may be proxied (it is also used by the schema export tool). 

This explanation is itself confounding. What does this mean?

constrained (optional) specifies that a foreign key constraint on the primary key of the mapped table references the table of the associated class.

kochcp · **Posted:** Mon Feb 18, 2008 10:36 am

my understanding is that constrained = true means that the class that you are referencing in that one-to-one relationship, a record must exist for each entry. So when you read an object in, it will be able to load a proxy for the relationship and when you then go to access it it will be able to run a sql call and read it in from the database.

It allows lazy loading in the case of one-to-one relationships. Where as lazy loading in one-to-one normally does not make sense because Hibernate usually overrides the collection methods with database calls and this isn't really possible in a one-to-one when you simply to a getObject() which is a relationship.

constrained = true is a guarantee to hibernate that the object exists so it doesn't error when it then goes to load it.

edit: Here is an article which probably explains it better

http://www.hibernate.org/162.html