Hi Professor, I'm having some problems creating the tables for a practice database. 

I checked out hibernate folder from the repository as a maven project. 

I went to persistence.xml and changed the information to my needs.  I created a practice database in postgres so I can play around with hibernate.  What I'm confused on is where is the actual command to create these tables to the database.  I run employeeTest and I get random errors.  Where is the creation of these tables taking place....and where do I click/run the code so my database can get populated by these tables. 

or... are we supposed to create the mappings, do the maven build --> process-classes -->  Get that script and execute it inside postgres? 

Can I use this instead...

In my previous hibernate experience, I was using new SchemaExport(config).create(true,true).  This was actually persisting the tables to the database.  Problem is, I needed a hibernate.xml file.

It was something like...

Configuration config = new Configuration();

config.addAnnotatedClass(Employee.class)

config.configure();

SessionFactory factory = config.buildSessionFactory();

new SchemaExport(config).create(true,true)

This would persist everything straight to the database without a script.

I never export the schema directly to the database because

1. without looking at the ddl it's hard to check whether the schema is correct or not.

2. the schema always needs some customization, e.g. set sequence range, add some additional constraints and so on.

3. you'll still need scripts to populate the tables, create indexes, stored procedures, views etc.

4. you can't run the code if the database already exists. e.g. it's usually very helpful to do bug fixes using a copy of the production database.

5. it's hard to keep track of the evolution of the schema without a script.

In fact, I can't think of any good reason to do it for a real world application. The only place for it seems to be in examples (which usually use an in-memory database) and maybe some unit tests.

And on top of that it's a Hibernate specific thing.

If you really want to do it, check out the 2nd to last solution here. It's basically uses SchemaExport with the JPA persistence unit configuration so you shouldn't need hibernate.cfg.xml.

Vanquish39 wrote:

Thank you for the great explanation.  It helped me clear things up.

Also professor, I'm still a little confused on the merge/persist thing.  Was the merge only used to regain a copy of the original persisted object?  Also, how can I persist an object, go from a persisted to detached state, and still access the object's values?  Is that what merge is for?

I don't quite understand your question. Basically in some circumstances there's a detached object, and then there's a managed copy loaded from the database. When you have two copies of an object (not just two references pointing to the same object), they can't magically become one, so merge() copies the states from the detached one to the managed one then saves it.

It sounds like there are two detached copies of the same object. You should change your code to avoid situations like this. In particular, don't use session attributes unless it's really necessary.

I don't know what you are talking about.
 

jonathankroening wrote:

cysun wrote:

I don't know what you are talking about.
 

Can you explain what this means then:  Found shared references to a collection: springmvc.model.Student.checked 

Based on the error message, it seems to mean that two objects (e.g. student1 and student2, or a managed student1 and a detached student1) are holding the same reference to a collection (i.e. checked).