Beginner needs help with modelisation issue

[balteo]

Hello, I have four tables in a relational database.

*First table, called "establishments", contains two fiels "db_establishment_id" and "db_establishment_name".
Here is a sample:

db_establishment_id(pk) db_establishment_name
---------------------------------------------
1 The black frog
2 The king's head
3 McDonalds

*Second table, called "categories", contains two fields "db_category_id" and "db_category_name"
Here is a sample:

db_category_id(pk) db_category_name
-----------------------------------
1 restaurant
2 pub

*Third table, called "establishments_qualifiers", contains two fields "db_establishment_id" and "db_id_category"
Here is a sample:

db_establishment_id(pk,fk) db_category_id(pk,fk)
--------------------------------------
1 2
2 2
2 1
3 1

*Fourth table, called "visitors_comments", contains two fields "db_establishment_id" and "db_comment"
Here is a sample:

db_establishment_id(pk) db_comment
-----------------------------------
2 nice pub
3 cheap food
3 yok!

My question is as follows: does one need four ejbs or can I have a single ejb here? (I'd rather have one). Ideally I'd like ONE ejb as follows:

-int establishement_id
-String establishment_name
-List categories
-List comments

How can I have a single ejb with the variables as above? Do I need BMP or CMP?

Thanks in advance and sorry for the length of my post,

Julien Martin.

[sesques]

Hi Julien,

My suggested Approach:
1. First of all decide do you require EJB or not?
if yes then..
You have 3 options for data access in j2ee.
1. CMP
2. BMP
3. Pure JDBC calls.

CMP:
CMP entity beans can span only a single database table. With the introduction of the EJB 2.0 standard, you can use CMR fields (Container Managed Relationships). A CMR allows a CMP entity bean to maintain a "relationship" to other CMP beans as long as these relationships are represented in the underlying database as foreign keys. CMR allows also relation tables (such as your establishments_qualifiers table). The CMP approach gives you 3 entity beans (establishments, categories and visitors_comments) with 2 relationships (one with a relation table).

BMP : With BMP, you can have a single entity bean to span your tables. But all the persistence will be managed by you. This is a tiresome, repetitive, and error-prone job, and it often sacrifices portability for the sake of performance.

JDBC calls : Write a session bean with pure JDBC calls allows you also to have a single bean span to your tables. Same opinion as BMP's.

Another solution will be using 3 CMP entity beans (as described above) , and one BMP entity bean which fields are the 3 other beans. In this case, the persistence will be managed by the container and you can use your tables through a single BMP bean.

Good luck

[balteo]

Sesques,
Thanks a lot for your lengthy reply!
Julien.

[triathlon98]

You could use a framework for this kind of thing.

Using Uni-d, you would define one table for this. The framework will then generate for your three CMP beans and session facade for you.

Problem is that Uni-d currently completey lacks documentation... Will have to work on that.

Joachim