Re: [Modeling-users] Re: attaching information to a relation

[Mario R. <ma...@ru...>]

On Mardi, ao=FB 5, 2003, at 00:01, Sebastien Bigaret wrote:
>
>         Hi Mario,
>
> Mario Ruggier <ma...@ru...> wrote:
>> On Lundi, ao=FB 4, 2003, at 20:49, Sebastien Bigaret wrote:
>>> Mario wrote:
>>>> just wondering how this can be best done... as a simple example,
>>>> assume a Person and an Address tables. A Person may have
>>>> more than one address, but each address may be categorized
>>>> as being "primary", "secondary", "private", "work", or anything
>>>> a priori unknown, Where does one best stick this information?
>>>
>>> I think this is what uml calls a "qualified association".
>>
>> Thanks for the explanation(s) below...
>> This would work just fine, if an Address belongs (exclusively)
>> to any one Person. What I am looking for is a way to reuse
>> the same Address for different people, but the type of
>> relation may be different each time. E.g.
>>
>> Mr. Dough's lives in a Villa, across from the beach (private Address)
>> Ladi Lean is the housemaid for Mr. Dough (work Address)
>> Mr. Dough's little sister spends summers at his villa (secondary=20
>> Address)
>> ...
>>
>> The most natural thing to me would be to add a "property"
>> to the association, if that'd be allowed. I guess one would have
>> to make an addditional association table for this?
>
> Okay, I didn't understand you properly. Now it's clear, you need
> qualified many-to-many associations. Here is a draft guideline for
> modeling a many-to-many relationship:

Very nice, thanks a lot. The association table is easy enough to
create (especially with the wonderfully expressive PyModel
Association statement). Thanks also for the sample code
to do the important manipulations. I will let you know what i
discover when doing such things... (have not played
with this part yet...). It would also certainly be great
to have a test/howto for this.

I have a question here: is it a good idea to re-use such
assoc tables to relate in a many-2-many way any arbitrary
pair of tables? An m2m relationship with a type seems to
me a frequently occurring situation, and it seems like a lot
of overhead to declare a dedicated assoc table per relation.
As an example, consider:

   Entity('User',
     properties=3D[
       AString('firstname',isRequired=3D1),
       AString('lastname',isRequired=3D1),
       ...
     ]
   ),

   Entity('Address',
     properties=3D[
       AString('street'),
       ...
     ]
   ),

   Entity('Organization',
     properties=3D[
       AString('name',isRequired=3D1),
       AString('type',isRequired=3D1),
     ],
   ),

   Entity('ManyToMany_Type',
     properties=3D[
       AString('type'),
     ]
   ),


  Association('User','ManyToMany_Type',
     relations=3D['addresses','user'],
     delete=3D['cascade','nullify'],
     keys=3D['FK_Address_id','id'] # what should this fk be named?
   ),

   Association('Address','ManyToMany_Type',
     relations=3D['users','address'],
     delete=3D['cascade','nullify'],
     keys=3D['FK_User_id','id'] # what should this fk be named?
   ),

  Association('User','ManyToMany_Type',
     relations=3D['organizations','user'],
     delete=3D['cascade','nullify'],
     keys=3D['FK_Organization_id','id'] # what should this fk be named?
   ),

   Association('Organization','ManyToMany_Type',
     relations=3D['users','organization'],
     delete=3D['cascade','nullify'],
     keys=3D['FK_User_id','id'] # what should this fk be named?
   ),

This, the same assoc table is used for the m2m relations
between User<>Address and User<>Organization.
Is this a good way?


Cheers, mario