| 
You have a N-to-1 relationship when an object of a class has an associated object of another class
(only one associated object) and several of this type of object can be linked to the same associated
object. From the other end of the relationship it is effectively a 1-N, but from the point of view of
the object in question, it is N-1. You can create the relationship in 2 ways depending on whether the
2 classes know about each other (bidirectional), or whether only the "N" side knows about the other
class (unidirectional). These are described below.
For this case you could have 2 classes,
User
and
Account
, as below.
so the
Account
class ("N" side) knows about the
User
class ("1" side), but not
vice-versa. A particular user could be related to several accounts. If you define the Meta-Data
for these classes as follows
<package name="mydomain">
<class name="User" identity-type="datastore">
<field name="login" persistence-modifier="persistent">
<column length="20" jdbc-type="VARCHAR"/>
</field>
</class>
<class name="Account" identity-type="datastore">
<field name="firstName" persistence-modifier="persistent">
<column length="50" jdbc-type="VARCHAR"/>
</field>
<field name="secondName" persistence-modifier="persistent">
<column length="50" jdbc-type="VARCHAR"/>
</field>
<field name="user" persistence-modifier="persistent">
</field>
</class>
</package>
This will create 2 tables in the database, one for
User
(with name
USER
), and one
for
Account
(with name
ACCOUNT
and a column
USER_ID
), as shown below.
Things to note :-
-
If you wish to specify the names of the database tables and columns for these classes, you can
use the attribute
table
(on the
class
element) and the attribute
name
(on
the
column
element)
-
If you call
PM.deletePersistent()
on the end of a 1-1 unidirectional relation without the
relation and that object is related to another object, an exception will typically be thrown
(assuming the RDBMS supports foreign keys). To delete this record you should remove the other
objects association first.
This relationship is described in the guide for 1-N
relationships. In particular there are 2 ways to define the relationship. The
first uses a Join Table to hold the relationship.
The second uses a Foreign Key in the "N" object to
hold the relationship. Please refer to the 1-N relationships bidirectional relations since they
show this exact relationship.
|
|
