Question

For example.

class One(models.Model):

     text=models.CharField(max_length=100)

class Two(models.Model):

     test = models.Integer()
     many = models.ManyToManyField(One, blank=True)

When I try save my object in admin panel, I take error such as:

“‘Two’ instance needs to have a primary key value before a many-to-many relationship can be used.”

I use django 1.3. I tried add AutoField to Two class, but it’s not work too.

This is my code.

from django.http import HttpResponse, HttpResponseRedirect
from django.shortcuts import render_to_response, redirect
from django.template import RequestContext
from django.core.urlresolvers import reverse
from project.foo.forms import FooForm
from project.foo.models import Foo
from project.fooTwo.views import fooTwoView

def foo(request, template_name="foo_form.html"):
    if request.method == 'POST':
        form = FooForm(data=request.POST)
        if form.is_valid():
            foo = Foo()
            foo.name = request.POST.get("name")
            foo.count_people = request.POST.get("count_people")
            foo.date_time = request.POST.get("date_time")
            foo.save()
            return fooTwoView(request)
    else:
        form = FooForm()

    return render_to_response(template_name, RequestContext(request, {
        "form": form,
    }))

P.S. I find my fail. It is in model. I used many-to-many in save method. I add checking before using, but it’s not help.

class Foo(models.Model):
    name = models.CharField(max_length=100, null=False, blank=False)
    count_people = models.PositiveSmallIntegerField()
    menu = models.ManyToManyField(Product, blank=True, null=True)
    count_people = models.Integer()
    full_cost = models.IntegerField(blank=True)

    def save(self, *args, **kwargs):
        if(hasattr(self,'menu')):
            self.full_cost = self.calculate_full_cost()
        super(Foo, self).save(*args, **kwargs)

    def calculate_full_cost(self):
        cost_from_products = sum([product.price for product in self.menu.all()])
        percent = cost_from_products * 0.1
        return cost_from_products + percent

I try hack in save method such as

if(hasattr(self,Two)):
        self.full_cost = self.calculate_full_cost()

This is help me, but i dont think that is the django way. What is interesting, that is without this checking admin panel show error, but create object. Now, if i select item from Two and save, my object does not have full_cost, but when i view my object, admin panel remember my choice and show me my Two item, what i select… I dont know why.

How do i save this?

Answer 1

There are quite a few problems with your code. The most obvious one are

1/ in your view, using a form for user inputs validation/sanitization/conversion then ignoring the santized/converted data and getting unsanitized inputs directly from the request. Use form.cleaned_data instead of request.POST to get your data, or even better use a ModelForm which will take care of creating a fully populated Foo instance for you.

2/ there’s NO implicit “this” (or “self” or whatever) pointer in Python methods, you have to explicitely use “self” to get at the instance attributes. Here’s what your model’s “save” method really do:

   def save(self, *args, **kwargs):
       # test the truth value of the builtin "id" function
       if(id):  
           # create a local variable "full_cost" 
           full_cost = self.calculate_full_cost()
       # call on super with a wrong base class
       super(Banquet, self).save(*args, **kwargs)
       # and exit, discarding the value of "full_cost"

Now with regard to your question: Foo.save is obviously not the right place to compute someting based on m2m related objects. Either write a distinct method that run the computation AND update Foo AND save it and call it after the m2m are saved (hint : a ModelForm will take care of saveing the m2m related objects for you), or just use the m2m_changed signal.

This being said, I strongly suggest you spend a few hours learning Python and Django – it will save you a lot of time.