How do I add a Foreign Key Field to a ModelForm in Django?

In regards to displaying a foreign key field in a form you can use the forms.ModelChoiceField and pass it a queryset.

so, forms.py:

class DocumentForm(forms.ModelForm):    class Meta:        model = Document    def __init__(self, *args, **kwargs):        user = kwargs.pop('user','')        super(DocumentForm, self).__init__(*args, **kwargs)        self.fields['user_defined_code']=forms.ModelChoiceField(queryset=UserDefinedCode.objects.filter(owner=user))

views.py:

def someview(request):    if request.method=='post':        form=DocumentForm(request.POST, user=request.user)        if form.is_valid():            selected_user_defined_code = form.cleaned_data.get('user_defined_code')            #do stuff here    else:        form=DocumentForm(user=request.user)    context = { 'form':form, }    return render_to_response('sometemplate.html', context,         context_instance=RequestContext(request))

from your question:

I know in a view you can use document.code_set (for example) to access the related objects for the current document object, but I'm not sure how to apply this to a ModelForm.

Actually, your Document objects wouldn't have a .code_set since the FK relationship is defined in your documents model. It is defining a many to one relationship to Code, which means there can be many Document objects per Code object, not the other way around. Your Code objects would have a .document_set. What you can do from the document object is access which Code it is related to using document.code.

edit: I think this will do what you are looking for. (untested)

forms.py:

class DocumentForm(forms.ModelForm):    class Meta:        model = Document        exclude = ('code',)    def __init__(self, *args, **kwargs):        user = kwargs.pop('user','')        super(DocumentForm, self).__init__(*args, **kwargs)        self.fields['user_defined_code']=forms.ModelChoiceField(queryset=UserDefinedCode.objects.filter(owner=user))        self.fields['unique_code']=forms.CharField(max_length=15)

views.py:

def someview(request):    if request.method=='post':        form=DocumentForm(request.POST, user=request.user)        if form.is_valid():            uniquecode = form.cleaned_data.get('unique_code')            user_defined_code = form.cleaned_data.get('user_defined_code')            doc_code = Code(user_defined_code=user_defined_code, code=uniquecode)            doc_code.save()            doc = form.save(commit=False)            doc.code = doc_code            doc.save()            return HttpResponse('success')    else:        form=DocumentForm(user=request.user)    context = { 'form':form, }    return render_to_response('sometemplate.html', context,         context_instance=RequestContext(request))

actually you probably want to use get_or_create when creating your Code object instead of this.

doc_code = Code(user_defined_code=user_defined_code, code=uniquecode)