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

[Sebastien B. <sbi...@us...>]

        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".
>=20
> 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.
>=20
> 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 Address)
> ...
>=20
> 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:

  - create entity PersonAdress --yes, this is the correlation table,

  - add attribute 'category' to it

  - add a to-many relationship from Person to PersonAdress, and its
    inverse (to-one): Person <---->> PersonAdress

  - add the two relationships: PersonAdress <<-------> Address

  - in Person, then add a method for getting addresses:

      def getAddresses(self, type=3DNone):
        if type is None:
          return self.valueForKeyPath('toPersonAddresses.toAddress')
        else:
          PAs=3D[pa for pa in self.getPersonAddresses() if pa.getType()=3D=
=3Dtype]
          addrs=3D[]
          [addrs.extend(pa.getPerson()) for pa in PAs]
          return addrs

   - add a similar one in Address if you need the inverse (from Address
     to Person),

   - be careful about setters, you need to manipulate the intermediate
     object, something like this:

       def addToAddresses(self, address, type):
         # you may also want to check that it's not already added
         # under an other category, or under the same category
         _pa=3DPersonAddress(type=3Dtype)
         self.editingContext().insert(_pa)
         self.addToPersonAddresses(_pa)
         _pa.setPerson(self)
         _pa.setAddress(address)
         address.addToPersonAddresses(_pa)

       def removeFromAddresses(self, address, type=3DNone):
         _pa=3D[pa in self.getPersonAddresses() if address =3D=3D pa.getAdd=
ress()]
         if type:
           _pa=3D[pa in _pa if pa.getType() =3D=3D type]
         if not _pa:
           raise ValueError, 'Cannot find address'
         # here you can get one or more PersonAddress, depending on
         # whether you allow the same Address to be registered under a
         # single category or not

         _pa=3D_pa[0] #or iterate on all, which could be handy if you want
                    #to remove all relations to 'address' when type=3D=3DNo=
ne
         self.removeFromPersonAddresses(_pa)
         _pa.getAddress().removeFromPersonAddresses(_pa)
         _pa.setAddress(None)
         _pa.setPerson(None)
         self.editingContext().remove(_pa)


  I didn't try it, though ;) I may create a special unittest suite for
  this, and use it to document many-to-many... If you try this, I'd like
  to hear from you, and I'll try to help to my best if have any problem.


        Cheers,

-- S=E9bastien.