What is the difference between lazy="true" and lazy="proxy" in nhibernate?
Best Answer
I suspect another way of thinking about it would be this.
class Foo
{
public virtual Bar SingleBar { get; set; }
public virtual ICollection<Bar> MultiBar { get; set; }
}
lazy="proxy" applies to single objects (ie foo.SingleBar)
lazy="true" applies to collections of objects (ie foo.MultiBar)
(You can't set lazy="proxy" to a collection, nor can you set lazy="true" to a single reference. Either will cause NH to throw a XmlSchemaException which is a little cryptic to beginners.)
Operationally they do the same abstract thing: when and only when the property is accessed does NHibernate hit the database and populate the property.
There is however a slight difference in the implementation due what is needed to fetch the objects (in the single case, the reference's id (Bar) was loaded with the parent entity (Foo). In the collection case, the ids are unknown and must be found in another table)
Save() takes a new object without an identifier and attaches it to the session. The object will be INSERT'd.
Update() takes an existing object that has an identifier but is not in the session and attaches it to the session. The object will be UPDATE'd.
SaveOrUpdate() looks at the identifier and decides what is necessary in the above.
SaveOrUpdateCopy() is special in that say you have two objects with the same identifier -- one in the session and one not. If you try and update the one not in the session an exception is thrown normally (you are now trying to attach two objects that represent the same persistent object to the session). SaveOrUpdateCopy() copies the non-session object state to the session object state.
I'm not sure how you are going to use NH, but for a lot of cases all you need is Save(). The session is doing ALL of the work necessary to know what has to be updated and simply Flush() or a Commit() does everything you need.
Best Answer
I suspect another way of thinking about it would be this.
(You can't set lazy="proxy" to a collection, nor can you set lazy="true" to a single reference. Either will cause NH to throw a XmlSchemaException which is a little cryptic to beginners.)
Operationally they do the same abstract thing: when and only when the property is accessed does NHibernate hit the database and populate the property.
There is however a slight difference in the implementation due what is needed to fetch the objects (in the single case, the reference's id (Bar) was loaded with the parent entity (Foo). In the collection case, the ids are unknown and must be found in another table)