roles can be found: http://pypi.python.org/pypi/roles/0.8
    the dci group (object composition) is: http://groups.google.com/group/object-composition

    some example code showing how to make it all "work" with django
is: http://groups.google.com/group/object-composition/browse_thread/threa...

    DCI is a very exciting architecture to me that extends the object
model &MVC/T to a more logical end and addresses a lot of complex
issues in design. The concepts have significantly altered my approach
to designing new systems. I really want to be able to make this work
cleanly with Django if at all possible. Appreciate any insights.

  -- Ben Scherrey

Could you post the full error message AND traceback you get when
trying to save your "roled" model instance please ? Django models have
a '._meta' attribute which stores, well, metadata about the model,
like fields, tablename etc, so the author's assumption (exposed in the
thread on the 'object-composition' group) that the problem is caused
by a class name change doesn't stand. I currently lack time to dig
into this DCI thing and python-roles implementation but given enough
context (=>traceback...) I might provide some hints.

HTH

I have tried to save the roled model and got [AttributeError: 'Person
+Carpenter' object has no attribute 'person_ptr_id'] as in traceback
below.

In [1]: from carpenter.models import Person, Carpenter
In [2]: jack = Person(name='Jack')
In [3]: Carpenter(jack)
Out[3]: <Person+Carpenter: Person+Carpenter object>
In [4]: jack.chop()
Out[4]: 'chop, chop'
In [5]: jack.save()
parent = <class 'carpenter.models.Person'>, field =
<django.db.models.fields.related.OneToOneField object at 0x21a4a50>
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call
last)

/home/juacompe/projects/dci/trunk/<ipython console> in <module>()

/home/juacompe/.virtualenvs/dci/lib/python2.6/site-packages/
Django-1.2.3-py2.6.egg/django/db/models/base.pyc in save(self,
force_insert, force_update, using)
    432         if force_insert and force_update:
    433             raise ValueError("Cannot force both insert and
updating in model saving.")
--> 434         self.save_base(using=using, force_insert=force_insert,
force_update=force_update)
    435
    436     save.alters_data = True

/home/juacompe/.virtualenvs/dci/lib/python2.6/site-packages/
Django-1.2.3-py2.6.egg/django/db/models/base.pyc in save_base(self,
raw, cls, origin, force_insert, force_update, using)
    474                 # this field). If so, fill it.

    475                 print 'parent = %s, field = %s' % (parent,
field)
--> 476                 if field and getattr(self,
parent._meta.pk.attname) is None and getattr(self, field.attname) is
not None:
    477                     setattr(self, parent._meta.pk.attname,
getattr(self, field.attname))
    478

AttributeError: 'Person+Carpenter' object has no attribute
'person_ptr_id'

The source code of the Person model and the Carpenter role can be
found in the link Ben gave above (http://www.google.com/url?
sa=D&q=http://groups.google.com/group/object-composition/browse_thread/
thread/fbb11a1e02b68de9&usg=AFQjCNFHv1njxL4B0SksOgPRFvj_hqhC0Q)

Any comments or suggestions would be really appreciated.

Thank you and best regards,
Chokchai Phatharamalai.

On Nov 23, 9:38 pm, bruno desthuilliers

Since - AFAICT - roles are about behaviour, using proxy inheritance
(http://docs.djangoproject.com/en/1.2/topics/db/models/#proxy-models)
could possibly cure the problem. Just add an inner "Meta" class in
your role model with "proxy = True" as class attribute (cf the above
link) and see if it works better.

My 2 cents.

Thank you very much!! Adding an inner Meta class works like charm.
Below is the working code with proving testcases
--------------------------
models.py
--------------------------
from django.db.models import Model, CharField
from django.db.models.base import ModelBase

from roles import RoleType

class ModelBaseRoleType(ModelBase, RoleType):
    """
    As every Django model which inherits from django.db.models.Model
already has
    ModelBase as its __metaclass__, applying a RoleType to it would
causes a
    conflict because a Python class can apply only 1 metaclass.

    In order to resolve the conflict, a new type named
ModelBaseRoleType is
    created.

    ModelBaseRoleType inherits from Django's ModelBase and Roles'
RoleType.

    ModelBaseRoleType also needs an inner Meta class to tell Django
that
    this will be a proxy class; otherwise, an AttributeError will be
raised
    when a *roled* model try to call save(). -- Thanks to Bruno
Desthuilliers
    for pointing this out. :)
    """
    # this metaclass avoids AttributeError in Django model
    class Meta:
        proxy = True

class Person(Model):
    name = CharField(max_length=20)

class Carpenter(object):
    __metaclass__ = ModelBaseRoleType

    def chop(self):
        return 'chop, chop'

-----------------
tests.py
-----------------
from carpenter.models import Person, Carpenter
from roles import RoleType, clone

from django.test import TestCase
from django.db import DatabaseError

class TestCarpenter(TestCase):
    def test_assign_role(self):
        """
        Role Carpenter is being assigned to a person
        """
        p = Person('juacompe')
        Carpenter(p)

    def test_save(self):
        jack = Person()
        jack.name = 'Jack'
        # applying carpenter role
        Carpenter(jack)
        self.failUnless(isinstance(jack, Person))
        self.failUnless(isinstance(jack, Carpenter))
        # a carpenter can chop
        jack.chop()
        # a person has name
        jack.name = 'Jack the giant killer'
        # django model can save
        jack.save()

Thank you very much!!

Best regards,
Chokchai P.

On Dec 3, 12:00 am, bruno desthuilliers

On 3 déc, 08:00, juacompe <juaco...@gmail.com> wrote:

Now beware, there _might_ be a couple other dark corners... I strongly
suggest you take a look at ModelBase implementation and possibly
extend your test cases, at least to make sure you spotted all possible
side-effects of the role+model combo.

I'll be out of town for a week, don't think i can code during that. so
my next post would be around the next weekend.

Have a good day!
Chokchai P.

On Dec 4, 12:19 am, bruno desthuilliers

I apply the Carpenter role (in the earlier post) in setUp of all test
classes in 2 small projects and add the test below in each of them.

-------------------
    def test_carpenter_role_applied(self):
        s = self.student
        self.failUnless(isinstance(s, Student))
        self.failUnless(isinstance(s, Carpenter))
        s.chop()
--------------------

Just to test that the model are properly roled and ran the whole test
suite to see if any of my assumptions were broken. All pass! Quite
happy with the result. :)

Thank again to Bruno for his kind advice. ;)

Best regards,
Chokchai P.

On Dec 4, 5:37 pm, juacompe <juaco...@gmail.com> wrote: