Design pattern to solve different validations on Django Models

Question

I'm trying to write application for my customer in Django 1.6.4

I've huge experience with Django but this case is quite hard for me to solve.

I'm using Substituting a custom user model feature and my Member class looks like (this is only simple snipplet):

class Member(AbstractBaseUser):
    ...
    date_of_birth = models.DateField()
    height = models.FloatField()
    ...
    REQUIRED_FIELDS = ['date_of_birth', 'height']

My Member can define multiple addresses with different countries. Each country has own specific address validations or extra fields, so I defined main abstract address and some subaddresses:

class COUNTRIES(object):
    POLAND = 'pl'
    GERMANY = 'de'
    @classmethod
    def to_choices(cls):
        return ((cls.POLAND, 'Poland'), (cls.GERMANY, 'Germany'))

class BaseAddress(models.Model):
    class Meta:
        abstract = True
    member = models.ForeignKet(settings.AUTH_USER_MODEL, related_name='+')
    country = models.CharField(max_length=2, choices=COUNTRIES.to_choices)

class AddressPL(BaseAddress):
    street = models.CharField(max_length=80)
    city = models.CharField(max_length=20)
    postal_code = models.CharField(max_length=6, validators=[RegexValidator('^\d{2}-\d{3}$'),])
    def save(self, *args, **kwargs):
        self.country = COUNTRIES.POLAND
        super(AddressPL, self).save(*args, **kwargs)

class AddressDE(BaseAddress):
    street = models.CharField(max_length=80)
    city = models.CharField(max_length=20)
    postal_code = models.CharField(max_length=6, validators=[RegexValidator('^\d{5}$'),])
    def save(self, *args, **kwargs):
        self.country = COUNTRIES.GERMANY
        super(AddressPL, self).save(*args, **kwargs)

As You can see the only change is in postal_code field in validators section. The only problem is that in Member I need to add something like this:

class Member(AbstractBaseUser):
    ...
    @property
    def addresses(self):
        from itertools import chain
        addresses_pl = AddressPL.objects.filter(member=self)
        addresses_de = AddressDE.objects.filter(member=self)
        return list(chain(addresses_pl, addresses_de))

based on 431628-how to combine 2 or more querysets

And my question is. Do You have any other idea to design this? I need to provide easy, scalable solution. Maybe any idea for using proxies?

Seems overkill to me to create another class just for a changing postal code. Maybe you would remove the validor in the model, and create a method that can raises ValidationError when the form is submitted. — David Dahan, Jun 11 '14 at 14:48

score 1 · Accepted Answer · answered Jun 11 '14 at 14:45

I think concrete inheritance could be a candidate for your problem:

class Member(AbstractBaseUser):
    ...
    date_of_birth = models.DateField()
    height = models.FloatField()
    ...
    REQUIRED_FIELDS = ['date_of_birth', 'height']

class BaseAddress(models.Model):
    # no abstract = True
    member = models.ForeignKey(settings.AUTH_USER_MODEL, related_name='adresses')
    country = models.CharField(max_length=2, choices=COUNTRIES.to_choices)

class AddressPL(BaseAddress):
    # your custom logic here

class AddressDE(BaseAddress):
    # your custom logic here

Then, if you want to add some data :

m = Member(yourargs)
m.save()

adress1 = AdressPL(yourargs)
adress1.member = m
adress1.save()

adress2 = AdressPL(yourargs)
adress2.member = m
adress2.save()

m = Member.objects.get(pk=m.pk)

m.adresses.all() # will output a list of BaseAdresse instances

This way, adding a new kind of address is just a matter of sublassing BaseAdress. No need to alter your user model.

The drawback of concrete inheritance is that it adds a join to your DB queries on inherited models.

Also, if you go this way, have a look at django-polymorphic, it's a Django app that provide useful method to deal with concrete inheritance.

Design pattern to solve different validations on Django Models

1 Answers1