Django ModelForm for Many-to-Many fields

I guess you would have here to add a new ModelMultipleChoiceField to your PizzaForm, and manually link that form field with the model field, as Django won't do that automatically for you.

The following snippet might be helpful :

class PizzaForm(forms.ModelForm):    class Meta:        model = Pizza    # Representing the many to many related field in Pizza    toppings = forms.ModelMultipleChoiceField(queryset=Topping.objects.all())    # Overriding __init__ here allows us to provide initial    # data for 'toppings' field    def __init__(self, *args, **kwargs):        # Only in case we build the form from an instance        # (otherwise, 'toppings' list should be empty)        if kwargs.get('instance'):            # We get the 'initial' keyword argument or initialize it            # as a dict if it didn't exist.                            initial = kwargs.setdefault('initial', {})            # The widget for a ModelMultipleChoiceField expects            # a list of primary key for the selected data.            initial['toppings'] = [t.pk for t in kwargs['instance'].topping_set.all()]        forms.ModelForm.__init__(self, *args, **kwargs)    # Overriding save allows us to process the value of 'toppings' field        def save(self, commit=True):        # Get the unsave Pizza instance        instance = forms.ModelForm.save(self, False)        # Prepare a 'save_m2m' method for the form,        old_save_m2m = self.save_m2m        def save_m2m():           old_save_m2m()           # This is where we actually link the pizza with toppings           instance.topping_set.clear()           instance.topping_set.add(*self.cleaned_data['toppings'])        self.save_m2m = save_m2m        # Do we need to save all changes now?        if commit:            instance.save()            self.save_m2m()        return instance

This PizzaForm can then be used everywhere, even in the admin :

# yourapp/admin.pyfrom django.contrib.admin import site, ModelAdminfrom yourapp.models import Pizzafrom yourapp.forms import PizzaFormclass PizzaAdmin(ModelAdmin):  form = PizzaFormsite.register(Pizza, PizzaAdmin)

Note

The save() method might be a bit too verbose, but you can simplify it if you don't need to support the commit=False situation, it will then be like that :

def save(self):    instance = forms.ModelForm.save(self)    instance.topping_set.clear()    instance.topping_set.add(*self.cleaned_data['toppings'])    return instance

I'm not certain I get the question 100%, so I'm going to run with this assumption:

Each Pizza can have many Toppings. Each Topping can have many Pizzas. But if a Topping is added to a Pizza, that Topping then automagically will have a Pizza, and vice versa.

In this case, your best bet is a relationship table, which Django supports quite well. It could look like this:

models.py

class PizzaTopping(models.Model):    topping = models.ForeignKey('Topping')    pizza = models.ForeignKey('Pizza')class Pizza(models.Model):         name = models.CharField(max_length=50)     topped_by = models.ManyToManyField('Topping', through=PizzaTopping)    def __str__(self):        return self.name    def __unicode__(self):        return self.nameclass Topping(models.Model):       name=models.CharField(max_length=50)    is_on = models.ManyToManyField('Pizza', through=PizzaTopping)    def __str__(self):        return self.name    def __unicode__(self):        return self.name

forms.py

class PizzaForm(forms.ModelForm):    class Meta:        model = Pizzaclass ToppingForm(forms.ModelForm):    class Meta:        model = Topping

Example:

>>> p1 = Pizza(name="Monday")>>> p1.save()>>> p2 = Pizza(name="Tuesday")>>> p2.save()>>> t1 = Topping(name="Pepperoni")>>> t1.save()>>> t2 = Topping(name="Bacon")>>> t2.save()>>> PizzaTopping(pizza=p1, topping=t1).save() # Monday + Pepperoni>>> PizzaTopping(pizza=p2, topping=t1).save() # Tuesday + Pepperoni>>> PizzaTopping(pizza=p2, topping=t2).save() # Tuesday + Bacon>>> tform = ToppingForm(instance=t2) # Bacon>>> tform.as_table() # Should be on only Tuesday.u'<tr><th><label for="id_name">Name:</label></th><td><input id="id_name" type="text" name="name" value="Bacon" maxlength="50" /></td></tr>\n<tr><th><label for="id_is_on">Is on:</label></th><td><select multiple="multiple" name="is_on" id="id_is_on">\n<option value="1">Monday</option>\n<option value="2" selected="selected">Tuesday</option>\n</select><br /> Hold down "Control", or "Command" on a Mac, to select more than one.</td></tr>'>>> pform = PizzaForm(instance=p1) # Monday>>> pform.as_table() # Should have only Pepperoniu'<tr><th><label for="id_name">Name:</label></th><td><input id="id_name" type="text" name="name" value="Monday" maxlength="50" /></td></tr>\n<tr><th><label for="id_topped_by">Topped by:</label></th><td><select multiple="multiple" name="topped_by" id="id_topped_by">\n<option value="1" selected="selected">Pepperoni</option>\n<option value="2">Bacon</option>\n</select><br /> Hold down "Control", or "Command" on a Mac, to select more than one.</td></tr>'>>> pform2 = PizzaForm(instance=p2) # Tuesday>>> pform2.as_table() # Both Pepperoni and Baconu'<tr><th><label for="id_name">Name:</label></th><td><input id="id_name" type="text" name="name" value="Tuesday" maxlength="50" /></td></tr>\n<tr><th><label for="id_topped_by">Topped by:</label></th><td><select multiple="multiple" name="topped_by" id="id_topped_by">\n<option value="1" selected="selected">Pepperoni</option>\n<option value="2" selected="selected">Bacon</option>\n</select><br /> Hold down "Control", or "Command" on a Mac, to select more than one.</td></tr>'

To be honest, I would put the many-to-many relation into the Pizza model. I think this closer to reality. Imagine a person that orders several pizzas. He wouldn't say "I would like cheese on pizza one and two and tomatoes on pizza one and three" but probably "One pizza with cheese, one pizza with cheese and tomatoes,...".

Of course it is possible to get the form working in your way but I would go with:

class Pizza(models.Model):    name = models.CharField(max_length=50)    toppings = models.ManyToManyField(Topping)