On Apr 19, 3:29 pm, Juanjo Conti <[EMAIL PROTECTED]> wrote:
> Hi all,
>
> I am worried about how to model the next scene (this is an example, but
> an appropriated one): In the model we have People, and there are
> different kind of people, let's say: Professors, Students and Other.
> Each People object has a 'type' attribute. Type can be P, S or O.
>
> So far, all right.
>
> Each people has an identifier that looks like: {LETTER}{NUMBER}. P1, P2,
> P3 and S1 are examples of valid identifiers.
>
> When you instance a People object you know its type but not its number:
>
> p = People(type="S")
>
> The question is, how can I handle this numbers in my model? The count
> should be dependent of the type.
>
> I am thinking about creating PostgreSQL sequences and use them to
> retrieve the propitiated number each time a People is instanced:
>
> ...
> if type == "S":
> self.number = seq_wrapper_s.next()
> elif typ == "P":
> self.number = seq_wrapper_p.next()
> else:
> self.number = seq_wrapper_o.next()
> ...
>
> But this will tie me to an specific data base engine. Is there a way of
> doing this task in Django in a independent-db-engine way?
You could add an IntegerField and a person_id property that's derived
dynamically to give you the P1, S2, etc. identifiers. Then, override
the save() method to manage the numeric sequence of those types.
class Person(models.Model):
numeric_id = models.IntegerField()
person_type = models.CharField() # model with options or
ForeignKey
class Meta:
unique_together = ((numeric_id, person_type),)
def _person_id(self):
return self.person_type + self.numeric_id
person_id = property(_person_id)
def save(self):
if not numeric_id:
# Need to assign the next numeric_id for this person_type
try:
last_of_this_type =
Person.objects.filter(person_type=self.person_type).order_by('-
numeric_id')[:1].get()
next_id = last.numeric_id + 1
except Person.DoesNotExist:
next_id = 1
self.numeric_id = next_id
super(Person, self).save()
As an unrelated side note: for the sake of following good convention,
consider using singular words instead of plural ones for your model
class names (i.e. Person instead of People.) In your example, I assume
that S1 means a single student and P2 means one professor. Then, it
makes sense to name your model Person instead of People.
-Rajesh D
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