Re: AW: mapping inheritance tree in one table

[David Rault]

Hi thomas

i understand perfectly the situation (i'm a toplink user, so i know the problem)
what puzzles me though is the need to have an attribute (which is what is told in the 3rd tutorial) in the objects
to keep a data that is inherently _in_ the object

my idea, which is submitted a PersistentField implementation for is to say

when reading from database, we use the current instantiation mechanism (which i trust is working :) )
but when writing to the db, we surely do not need an attribute to contain the class name do we ?

we just need a field to store the information : let's keep the special field name "ojbConcreteClass", that's fine with me
and we need to tell OJB to call getClass().getName() on the to-be-stored object to know what to write

to be more precise : here's my (very classic) test classes :

class ACCOUNT {
int id;
double value;
string number
}

class SAVING_ACCOUNT extends ACCOUNT {
double rate;
}

class BORROWING_ACCOUNT extends ACCOUNT {
double rate;
int duration
}

i ve got a single table :
create table ACCOUNTS (
id integer primary key,
concreteClass varchar,
value number,
numero varchar,
rate number,
duration integer,
}

i simply do not want (i believe i don't _need_ it either) to add the "String ojbConcreteClass" attribute in ACCOUNT class

here's my repository.xml:
<class-descriptor class="ACCOUNT" table="ACCOUNTS">
<extent-class class-ref="SAVINGS_ACCOUNT" />
<extent-class class-ref="BORROWING_ACCOUNT" />
<field-descriptor id="1" name="id" column="ID" jdbc-type="INTEGER" primarykey="true" autoincrement="true" />
<field-descriptor id="2" name="ojbConcreteClass" column="CONCRETECLASS" jdbc-type="VARCHAR" />
<field-descriptor id="3" name="number" column="NUMBER" jdbc-type="VARCHAR" />
<field-descriptor id="4" name="value" column="VALUE" jdbc-type="DOUBLE" />
</class-descriptor>

<class-descriptor class="SAVINGS_ACCOUNT" table="ACCOUNTS">
<field-descriptor id="5" name="rate" column="RATE" jdbc-type="DOUBLE" />
</class-descriptor>

<class-descriptor class="BORROWING_ACCOUNT" table="ACCOUNTS">
<field-descriptor id="5" name="rate" column="RATE" jdbc-type="DOUBLE" />
<field-descriptor id="6" name="duration" column="DURATION" jdbc-type="INTEGER" />
</class-descriptor>


so reading from the database is easy : i've got the ojbConcreteClass field descriptor with its value
the setting of the attribute is useless, i don't need/want to save it, my implementation just do nothing
and when writing, my PersistentField implemenation says that for the ojbConcreteClass field descriptor the value is the result of getClass().getName()

is it clearer ?
and then is it wrong ? (i obviously might have missed something)
David