Queston about CMR and corresponded database table.

Question

Hi:
For example, If I have a EntityA with cmp field named "EntityA_ID" which is
the primary key field, and have a EntityB with cmp field named "EntityB_ID" which
is the primary key field. The relation of EntityA and EntityB is 1:1.
But the container will create table EntityA_Table and EntityB_Table:
CREATE TABLE EntityA_Table (ID varchar (50) PRIMARY KEY,_EntityA_EntityB
varchar(50))
CREATE TABLE EntityB_Table (ID varchar (50) PRIMARY KEY,_EntityB_EntityA
varchar(50))
But the column for relation is as same as the primary key column, the question
is below:
Q1: Does the cmp field and cmr field can not be the same database table column?
If so, it will waste the disk space.
Q2: which one is more common or more better?
1)create database table first and create CMP based on table.
2)create CMP first and create table from EJB descriptor thought EJB container
when deploying.
3)create CMP first and create table from EJB descriptor by manual.
Regards!
John Lee

Answer 1

The relation of EntityA and EntityB is 1:1.
But the container will create table EntityA_Table and EntityB_Table:
CREATE TABLE EntityA_Table (ID varchar (50) PRIMARYKEY,_EntityA_EntityB
varchar(50))
CREATE TABLE EntityB_Table (ID varchar (50) PRIMARYKEY,_EntityB_EntityA
varchar(50))
for 1:1 relationships, only one table is required to contain the Foreign
Key.
But the column for relation is as same as the primary key column, thequestion
is below:
Q1: Does the cmp field and cmr field can not be the same database tablecolumn?
If so, it will waste the disk space.The primary key cmp field column cannot be the same database column as the
foreign key cmr field column because the primary key cannot change value
once it is set whereas the foreign key may change according to how
relationships are made and broken.
Q2: which one is more common or more better?
1)create database table first and create CMP based on table.
2)create CMP first and create table from EJB descriptor thought EJBcontainer
when deploying.
3)create CMP first and create table from EJB descriptor by manual.
At development time allowing the container to create the tables is
convenient.
It is not recommended to rely on this mechanism for production use.
Generally,
one wants much more control over the database tables in production than the
simple things that the container does when it creates 'default' tables.
-thorick