Question

I have a model w/ a manytomany relation to another model. They both have forms associated with them, the latter has a formset.

 class Foo(models.Model):
   name = models.CharField(max_length=20)
   bars = models.ManyToManyField("Bar",blank=True,null=True)

 class Bar(models.Model):
   name = models.CharField(max_length=20)

 class FooForm(ModelForm):
   class Meta:
     model = Foo

 class BarForm(ModelForm):
   class Meta:
     model = Bar

 BarFormSet = modelformset_factory(Bar,form=BarForm,extra=2)

In my view/template the standard ManyToManyField widget is replaced with the formset. Thus, I have to manually associate the instances of Bar specified in that formset with Foo’s ManyToManyField. I am doing this in Foo’s clean method:

 def clean(self,*arg,**kwargs):
   cleaned_data = self.cleaned_data
   # barSubFormInstance is the BarSubForm that is displayed in my view
   if barFormSetInstance.is_valid(): 
     barInstances = barFormSetInstance.save()
     cleaned_data["bars"] = barInstances
   return cleaned_data

This almost works. The problem is that it sets Foo.bars to the set of changed forms within the formset. Thus if I add one bar to my foo, then reload the form and add a second bar, the foo winds up only having that second bar.

According to the Django documentation:

The save() method returns the instances that have been saved to the database. If a given instance’s data didn’t change in the bound data, the instance won’t be saved to the database and won’t be included in the return value…

So I understand why my code is failing. I just don’t know what to do about it. What can I pass to cleaned_data[“bars”] that will add the newly modified forms but not remove the existing ones?

Many thanks for your help.

Answer 1

For what it’s worth, there was an error in the above comment. And some other details I left out.

The appropriate code looks like this:

def clean(self,*arg,**kwargs):
   cleaned_data = self.cleaned_data
   # barFormSetInstance is the BarFormSet that is displayed in my view
   # it's already been validated by the time this function is called
   barInstances = [form.save() for form in barFormSetInstance if form.cleaned_data]
   cleaned_data["bars"] = barInstances
   return cleaned_data

I also had to ensure that “blank” and “null” were both set to True for the ManyToManyField. (I’m not sure why I had to do that instead of overriding that field’s is_valid() method to just return True).