How make this special many2many fields validation using Django ORM?

I have the folowing model:

class Step(models.Model):

    order = models.IntegerField()
    latitude = models.FloatField()
    longitude = models.FloatField()
    date = DateField(blank=True, null=True)


class Journey(models.Model):

    boat = models.ForeignKey(Boat)
    route = models.ManyToManyField(Step)
    departure = models.ForeignKey(Step, related_name="departure_of", null=开发者_如何学CTrue)
    arrival = models.ForeignKey(Step, related_name="arrival_of", null=True)

I would like to implement the following check:

        # If a there is less than one step, raises ValidationError.

        routes = tuple(self.route.order_by("date"))

        if len(routes) <= 1:
            raise ValidationError("There must be at least two setps in the route")

        # save the first and the last step as departure and arrival
        self.departure = routes[0]
        self.arrival = routes[-1]

        # departure and arrival must at least have a date
        if not (self.departure.date or self.arrival.date):
            raise ValidationError("There must be an departure and an arrival date. "
                                  "Please set the date field for the first and last Step of the Journey")

        # departure must occurs before arrival    
        if not (self.departure.date > self.arrival.date):
            raise ValidationError("Departure must take place the same day or any date before arrival. "
                                  "Please set accordingly the date field for the first and last Step of the Journey")

I tried to do that by overloading save(). Unfortunately, Journey.route is empty in save(). What's more, Journey.id doesn't exists yet. I didn't try django.db.models.signals.post_save but suppose it will fail because Journey.route is empty as well (when does this get filled anyway?). I see a solution in django.db.models.signals.m2m_changed but there are a lot of steps (thousands), and I want to avoid to perform an operation for every single of them.

---

If you're running the latest and greatest Django, have a look at this http://docs.djangoproject.com/en/dev/ref/models/instances/#id1

Basically, it's possible to validate Models in the same way as Forms. While I've never used it myself, it looks as a good fit to what you're trying to do.

---

Eventually, I had to write a validation for every form creating this object. knutin's solution would have been great but I run Django 1.1.

Anyway, Django let you overload the admin validation quite easily so I did :

class JourneyAdminForm(forms.ModelForm):

    class Meta:
        model = Journey

    def clean_route(self):
        """
            Ensure a Journey includes at least 2 dated steps, 
            departure starting before arrival.
        """

        # must use getlist as self.data is not a dict but a querydict
        routes = self.data.getlist("route")

        if len(routes) <= 1:
            raise ValidationError("There must be at least two setps in the route")

        departure = Step.objects.get(id=routes[0])
        arrival = Step.objects.get(id=routes[-1])
        self.data["departure"] = departure.id
        self.data["arrival"] = arrival.id


        if not (departure.date and arrival.date):
            raise ValidationError("There must be an departure and an arrival date. "
                                  "Please set the date field for the first and last Step of the Journey")

        if not (departure.date <= arrival.date):
            raise ValidationError("Departure must take place the same day or any date before arrival. "
                                  "Please set accordingly the date field for the first and last Step of the Journey")

        return self.cleaned_data["route"]

class JourneyAdmin(admin.ModelAdmin):

    exclude = ("arrival", "departure")
    form = JourneyAdminForm

admin.site.register(Journey, JourneyAdmin)

As a bonus, ValidationError messages are displayed as a feedback for the user when submitting the form.