HOWTO: Use BC4J With or Without DB Triggers

Question

This HowTo describes how to use BC4J, database sequences and triggers
and what are the ramifications.
INTRODUCTION
BC4J has the ability to work with database sequences in order to obtain a
unique value when inserting records. BC4J also has the ability to
work either with a 'before insert' trigger which automatically creates
a new unique value for the primary key or without a trigger. When not using
a database trigger, BC4J also has the ability to obtain the sequence value
and set the primary key value.
Before discussing the ramifications of using one approach or the other, let's
show examples of how to use both approaches:
BC4J & sequences WITH a database trigger
and
BC4J & sequences WITHOUT a database trigger
HOWTO DEMONSTRATION STEPS
To illustrate both scenarios a simple database setup script is provided which
creates two tables:
CUSTOMER_NT which DOES NOT have a before insert trigger and
CUSTOMER_WT which DOES have a trigger.
Database Install Script:
<code>
drop trigger customer_insert_trigger;
drop table customer_wt;
drop table customer_nt;
drop sequence customer_wt_seq;
drop sequence customer_nt_seq;
create sequence customer_wt_seq start with 1;
create sequence customer_nt_seq start with 101;
create table customer_wt(
id number,
name varchar2(30),
constraint
customer_wt_pk
primary key (id)
create table customer_nt(
id number,
name varchar2(30),
constraint
customer_nt_pk
primary key (id)
prompt Inserting data...
insert into customer_wt (id, name)
values (customer_wt_seq.nextval, 'Mickey');
insert into customer_wt (id, name)
values (customer_wt_seq.nextval, 'Goofy');
insert into customer_nt (id, name)
values (customer_nt_seq.nextval, 'Daffy');
insert into customer_nt (id, name)
values (customer_nt_seq.nextval, 'Porky');
commit
prompt Creating trigger
create trigger customer_insert_trigger
before insert on customer_wt for each row
begin
select customer_wt_seq.nextval into :new.id from dual ;
end;
</code>
The next step is to create the DEFAULT Entity Objects and View Objects using
the Business Components Wizard.
USING BC4J WITH A DATABASE TRIGGER
Let's modify the entity object CustomerWt so it can use the database trigger.
Edit the entity object CustomerWt by right-clicking in the navigator.
Click on the 'Attribute Settings' tab and edit the ID attribute.
- Uncheck 'Mandatory'checkbox. This allows you to insert without a value for the primary key
- Check 'Refresh after Insert'. This obtains the value from the database generated by the trigger.
- Check 'Updateable While New'. Id is only updateable when inserting.
Click finish to complete the wizard. Save all and recompile the project.
Now let's test our work.
In the navigator right-click the application module and select 'Test..'. This will launch
BC4J's built in tester. Connect to the application.
In the tester double-click the CustomerWtView view object to run a test edit form.
After the edit form renders, navigate through the existing records using the navigate
buttons on the edit form. Now let's insert a record to execute the trigger.
click on the '+' button to insert a record. Enter a value in the 'Name' field and commit the change.
Observe that a new value has automatically been inserted into the Id field.
That's it! You have successfully used BC4J and a database trigger.
Now let's try it without a trigger..
USING BC4J WITHOUT A DATABASE TRIGGER
Now edit the entity object CustomerNT so it doesn't need a database trigger.
Similar to before, edit the entity object CustomerNt by right-clicking in the navigator.
Click on the 'Attribute Settings' tab and edit the ID attribute.
- Uncheck 'Mandatory'checkbox.
- Check 'Updateable While New'.
An additional step is also required. The Create method will have to be modified to extract
the value of the sequence.
In the Edit EntityObject Wizard click the Java tab and select Create method and click Finish.
The create method is generated in your Java fil e. In the Workspace view of the Navigator,
expand the CustomerNt entity object in the navigator. Double-click
CustomerNtImpl.java to open it in the Source Editor. In the Structure pane, double-click
create(AttributeList). Modify the Create method so it looks like this:
<code>
public void create(AttributeList attributeList) {
super.create(attributeList);
SequenceImpl s = new SequenceImpl("customer_nt_seq", getDBTransaction());
Integer next = (Integer)s.getData();
setId(new Number(next.intValue())); }
</code>
Save and compile the project.
Now test the ViewObject CustomerNtView using the tester as before.
In the edit form of CustomerNTView click on the '+' to insert a record. Observe that
just as before a new value has automatically been inserted in the ID field!
TO USE A DB TRIGGER OR NOT TO USE A DB TRIGGER.
Using a Database trigger sometimes preferable if you have non BC4J applications
also sharing the database. In this case it is still safest to just let the database
update it's own primary keys.
If you don't have any other non-BC4J applications sharing the database, then not using
a database trigger is perfectly acceptable and can have slightly better performance.
The important thing to remember is that the option is yours to use either approach!
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Answer 1

Thank you for the reply Jonathon. I am using a ViewObject which
consist of several tables. I haven't tried the DB trigger
approach but just using the BC4 approach in overriding the
create method.
Here is the parent class create as a part of the
FasNameImpl.java file which does the job correctly.
public void create(AttributeList attributeList) {
super.create(attributeList);
SequenceImpl l_seq = new SequenceImpl
("SEQ_CUSTOMER_ID",getDBTransaction());
Integer l_next = (Integer)l_seq.getData();
setCustomerId(new Number(l_next.intValue()));
This is when I triedpassing the value to the child table. But I
can't figure it out. I think the link is working fine if I had a
ViewLink deployed but it doesn't look like it's doing the job
for ViewObject.
I am trying to call the childclass.method
(FasCustomer.setCustomerId(l_next);
But I am getting error.
Thanks a lot for your suggestions,
Kamran
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