Object Relational Mappers: Difference between revisions

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== Java: Hibernate ==
== [[Hibernate|Java: Hibernate ]] ==


* If you specify a one-to-many or many-to-many mapping, be sure to use the same mapping as the Java collection interface. A Set mapping won't work with a List collection, for example.
* Hibernate is THE reference ORM, the most advanced solution available right now. It is extremely powerful although it takes quite some time to fully learn it (especially its model mappings, which can be complex since they allow almost any kind of mapping).
 
* In Hibernate, lazy loading is on by default for collections. This means the collections are fetched from the database only when they are used (never if they are not used). '''Warning:''' if the loading appears when the Hibernate session is already closed, it will '''fail.'''
 
* If you have an object and a many-to-one mapping on this object (such as Item that belongs to a Category), and you happen to have the ID of the Category but not the associated Java object, in order to store the item on the database the fastest way is to create a new virtual Category and call setID(known_id) on this newly created object. Then, once you save the object, the virtual Category will not be saved (updated) as it already exists. For this to happen (eg, for the object not to be overriden), you must use the ID of the Object as the equals() method. '''This is not recommended.''' Note that by default Hibernate distinguishes between objects by using their addresses in memory.
 
* '''Update:''' The previous remark would work, '''but is not the recommended way.''' The correct way is to use session.load(Category.class, category_id) and use a proxy. This means the object won't actually be fetched from the DB, but you'll be able to use it as a Java object. This is nice. By default, a mapping uses a proxy.

Latest revision as of 10:44, 19 May 2008

Java: Hibernate

  • Hibernate is THE reference ORM, the most advanced solution available right now. It is extremely powerful although it takes quite some time to fully learn it (especially its model mappings, which can be complex since they allow almost any kind of mapping).
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