AW: [Objectbridge-developers] Sample Many-to-Many Mapping XML

[Mahler T. <tho...@it...>]

Hi Sasha, hi all,

> -----Urspr=FCngliche Nachricht-----
> Von: Sasha Haghani [mailto:sha...@br...]
> Gesendet: Dienstag, 29. Mai 2001 01:21
> An: 'obj...@li...'
> Betreff: [Objectbridge-developers] Sample Many-to-Many Mapping XML
>=20
>=20
> Hi Thomas et al,
>=20
> I have a question about how to implement a M-M relationship=20
> using OJB.  I've
> included a sample mapping which uses three objects, User, Role &
> UserRoleRelationship.  User and Role each have a collection of
> UserRoleRelationship objects.  And UserRoleRelationship has a=20
> single ID and
> reference field for both User and Role respectively.
>=20
> Can someone verify that this mapping is correct.  Is there anyway to
> simplify it further?  Is a UserRoleRelationship object=20
> absolutely necessary?

Right now this is exactly the thing you have to do! You need three =
tables to
decompose m-n into two 1-n relationships. Thus you also need three =
classes.
This approach is also suggested by the gurus: see
ftp://members.aol.com/kgb1001001/Chasms/chasms.pdf.

As there have been requests to have a standard mapping for m-n =
relationships
I'm thinking of having some support for m-n in OJB at some later point =
in
time ...

> Note that I've defined both the userid and roleid columns as=20
> primary keys
> for the UserRoleRelationship (user_role table) -- is that valid?
>=20

Of course the combination of userid and roleid must be unique. If you =
use
them as PK you don't even need a separate ID column.

> Finally, are my auto.* settings configured correctly?  I'd=20
> like the deletion
> of a User or Role to delete the object and any corresponding
> UserRoleRelationships, both *NOT* the other side of the=20
> relation (i.e.:
> typical Many-to-Many behaviour).  Retrieving any object=20
> should retrieve any
> dependent object (User gets related UserRoleRelationships and=20
> Roles, etc.).
>=20
> Any help on this or any alternative mapping samples are much=20
> appreciated.
> Thanks.  The mapping sample XML is below...
>=20
> Sasha Haghani
> Toronto, Canada.
>=20
> 	<ClassDescriptor id=3D"1">
> 		<class.name>User</class.name>
>       	<table.name>user</table.name>
> 		<FieldDescriptor id=3D"1">
>          		<field.name>userid</field.name>
>          		<column.name>userid</column.name>
>          		<jdbc_type>CHAR</jdbc_type>
>          		<PrimaryKey>true</PrimaryKey>
>       	</FieldDescriptor>
>       	<FieldDescriptor id=3D"2">
>          		<field.name>username</field.name>
>          		<column.name>username</column.name>
>          		<jdbc_type>CHAR</jdbc_type>
>       	</FieldDescriptor>
>     		<CollectionDescriptor id=3D"1">
>          		<cdfield.name>roles</cdfield.name>
>          		<items.class>UserRoleRelationship</items.class>
>          		<descriptor_ids>1</descriptor_ids>
>          		<auto.retrieve>true</auto.retrieve>
>          		<auto.update>true</auto.update>
>          		<auto.delete>true</auto.delete>
>       	</CollectionDescriptor>
>    	</ClassDescriptor>
>=20
> 	<ClassDescriptor id=3D"2">
> 		<class.name>UserRoleRelationship</class.name>
>       	<table.name>user_role</table.name>
> 		<FieldDescriptor id=3D"1">
>          		<field.name>userid</field.name>
>          		<column.name>userid</column.name>
>          		<jdbc_type>CHAR</jdbc_type>
>          		<PrimaryKey>true</PrimaryKey>
>       	</FieldDescriptor>
>       	<FieldDescriptor id=3D"2">
>          		<field.name>roleid</field.name>
>          		<column.name>roleid</column.name>
>          		<jdbc_type>CHAR</jdbc_type>
>          		<PrimaryKey>true</PrimaryKey>
>       	</FieldDescriptor>
>       	<ReferenceDescriptor id=3D"1">
>          		<rdfield.name>user</rdfield.name>
>          		<referenced.class>User</referenced.class>
>          		<descriptor_ids>1</descriptor_ids>
>          		<auto.retrieve>true</auto.retrieve>
>          		<auto.update>false</auto.update>
>          		<auto.delete>false</auto.delete>
>       	</ReferenceDescriptor>
>       	<ReferenceDescriptor id=3D"2">
>          		<rdfield.name>role</rdfield.name>
>          		<referenced.class>Role</referenced.class>
>          		<descriptor_ids>2</descriptor_ids>
>          		<auto.retrieve>true</auto.retrieve>
>          		<auto.update>false</auto.update>
>          		<auto.delete>false</auto.delete>
>       	</ReferenceDescriptor>
>    	</ClassDescriptor>
>=20
> 	<ClassDescriptor id=3D"3">
> 		<class.name>Role</class.name>
>       	<table.name>role</table.name>
> 		<FieldDescriptor id=3D"1">
>          		<field.name>roleid</field.name>
>          		<column.name>roleid</column.name>
>          		<jdbc_type>CHAR</jdbc_type>
>          		<PrimaryKey>true</PrimaryKey>
>       	</FieldDescriptor>
>       	<FieldDescriptor id=3D"2">
>          		<field.name>rolename</field.name>
>          		<column.name>rolename</column.name>
>          		<jdbc_type>CHAR</jdbc_type>
>       	</FieldDescriptor>
>     	<CollectionDescriptor id=3D"1">
>          		<cdfield.name>roles</cdfield.name>
>          		<items.class>UserRoleRelationship</items.class>
>          		<descriptor_ids>2</descriptor_ids>
>          		<auto.retrieve>true</auto.retrieve>
>          		<auto.update>true</auto.update>
>          		<auto.delete>true</auto.delete>
>       	</CollectionDescriptor>
>    	</ClassDescriptor>=09
>=20

Yes this looks good! If a user is deleted all dependent =
UserRoleRelationship
entries are deleted, but role entries are not touched, as the =
auto.update
and auto.delete flags are set to false!

Sasha, I have a question: could you send me the sources for classes =
User,
Role and UserRoleRelationship?
Until now I have no sample for a m-n mapping and would like to include
something simple into our test package for demonstration of mapping
techniques.

thanks,

Thomas


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>=20