Using a CMP Entity Bean local stub as a field of another CMP Entity Bean

Question

Hello,
Is it possible to implement a field of a CMP Entity bean as another CMP Entity bean and how is it done?
I've seen a pseudo code for this in Ed Roman's Mastering EJBs, second edition, but I can't seem to get it to work (pages: 330 - 1:1 using CMP and 339 - fake M:N using CMP).
I'm using SUN ONE Application Sever 7. Is this server capable of this?
I'm trying to implement a fake M:N relationship using 3 beans: 2 for each side of the relationship and one as the "bridge" table.
For example, the two beans on each side of the relationship are SubscriberBean, SubscriptionBean and the "bridge: bean is SubscriberSubscriptionBean. The SubscriberSubscriptionBean has two fields: SubscriberLocal stub and SubscriptionLocal stub.
Please let me know if you need more information to answer this question.
Thanks.
Nikola

Answer 1

Im sorry but i dont know about the example you are talking about. I kinda learn
all those techniques from forums, articles and tutorials because book often suffer from
not having the information im mostly looking for.
As far as i understand you, you want to implement a bridge been, which is representing a row in a join table. So that if one side of the relation is deleted the join-table entry (your bridge-CMP-Bean) is cascaded. Right?
First of all the simple part: (My approach)
- The joint table is foreign keys only - without a relation description. -
In this case you dont have to implement a bridge bean. Because it just wouldnt represent anything of sense.
Lets think of an entity/table USER whith the columns name (PRIMARY KEY), and prename.
Our second entity/table is ADDRESS with the columns road (PRIMARY KEY) and housenr.
The join table is simply: USER_ADDRESS with fk_name (FOREIGN KEY) and fk_road (FOREIGN KEY) both on CASCADE DELETE. So if the address is deleted the mapping entry is deleted, too same for the user part:
USER -> USER_ADDRESS <- ADDRESS.
Our entity Beans are called User and Address in class-names JNDI-names and names.
Now we want to create the CMR mapping so we can access the addresses of a user from the user bean directly. The methods on the user side are:
public abstract Collection getAddresses();
and
public abstract void setAddresses(Collection new_addresses);
the xdoclet comments on the User side are (for the getter only)
* @ejb.interface-method
* @ejb.relation
*                name = "User-has-Addresses"
*                role-name = "User-Addresses"
*                target-ejb = "Address"
*                target-multiple = "true"
* @sunone.relation
*                column="USER_ADDRESS.fk_name"               
*                target="USER_ADDRESS.fk_road"               
public abstract Collection getAddresses();
for the other side of the relation we define in the Address-Entity
public abstract Collection getUsers();
and
public void setUsers(Collection users);
the xdoclet comments on the Address side are (for the getter only)
* @ejb.interface-method
* @ejb.relation
*                name = "User-has-Addresses"
*                role-name = "Address-User"
*                target-ejb = "User"
*                target-multiple = "true"
* @sunone.relation
*                target="USER_ADDRESS.fk_road"               
*                column="USER_ADDRESS.fk_name"
As we dont want the user or address to be deleted if the other side of the relation is deleted we dont specify cascade-delete="yes" in the ejb.relation namespace.
The sun-cmp-mappings.xml should now look like this:
(For the User - side)
<!-- Relationship User-has-Addresses, role User-Addresses -->
<cmr-field-mapping>
<cmr-field-name>addresses</cmr-field-name>
<column-pair>
<column-name>USER_ADDRESS.fk_name</column-name>
<column-name>USER_ADDRESS.fk_road</column-name>
</column-pair>
</cmr-field-mapping>
and similar on the Address-Side:
<cmr-field-mapping>
<cmr-field-name>users</cmr-field-name>
<column-pair>
<column-name>USER_ADDRESS.fk_road</column-name>
<column-name>USER_ADDRESS.fk_name>/column-name>
</column-pair>
</cmr-field-mapping>
Dont forget that all elements in the Collection must be of the right Interface-type.
First of all the harder part:
Now what you might want is when the relation has some information specified like user live at address and is tenant or facility manager.
The, in the first step you will have to implement the UserAddressRelation entity which will have to CMR fields of the 1:N type.(Just as you wish)
Lets think of the example above extended by the relation type. Our relation bean is named UserAddressRelation.
Now User has the methods as above but the classes in the Collection must now be of the UserAddressRelationLocal/Remote interface and not AddressLocal/Remote-interface.
You will have to change the xdoclet comment to:
* @ejb.interface-method
* @ejb.relation
*                name = "User-has-Addresses"
*                role-name = "User-Address"
*                target-ejb = "UserAddressRelation"
*                target-multiple = "true"
* @sunone.relation
*                target="USER_ADDRESS.fk_name"               
*                column="USER.name"
So we now dont reference the other side bean directly. We reference the relation EntityBean. Do the similar changes on the other side.
In particular you will have to specify 4 CMR mappings now:
1. User to UserAddressRelation 1:N
2. UserAddressRelation N:1
3. Address to UserAddressRelation 1:N
4. UserAddressRelation N:1
Which i dont want to explain in detail now because its kinda all the same as above.
Now you cann access the Addresses of a user in the way.
UserLocal.getAddresses(); <- !you get the mappings!
UserLocal.getAddresses().item(0).getAddress() <- you get the address (this is dirty coding just for understanding)
UserLocal.getAddresses().item(0).getUserRole() <. you get the role of the user at this address.
Hope this helped you. You are welcome to ask any detailed question.