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I have recently been working more and more with django forms. In previous projects we decided not to use them mostly cause we couldn’t get them to do what we wanted them to do. In retrospect it was more of a lack of understanding how they worked and insufficient time to sit down and learn them.

Earlier in the week I was working with a model based form with a foreign key to another model. Problem I ran across was that I wanted said form to be displayed on the page, and also post the proper foreign key id without the user selecting it ( foreign keys are selected through a select box which can get quite large ). The page in which this form would be displayed would always know the foreign key id, so there would never need to be a situation where the form would ever need to show the user a select box.

After much trial and error, here is what I came up with.

from django.forms import ModelForm
from django import forms
from models import Thread
from models import Post

class ThreadForm(ModelForm):
    class Meta:
        model = Thread
        fields = ('Message', 'user')

class PostForm(ModelForm):
    class Meta:
        model = Post
        fields = ( 'Message', 'user')

The Post in question would normally have a foreign key in this instance to Thread, but we exclude it from the form so that it is not validated against or displayed to the user.

    <form action="/post/{{ thread.pk }}/" method="post">
        {{ form.as_p }}
        <input type="submit" value="Submit" />
    </form>

In the actual form posting we are using the threads key in the url that is receiving the new post, If you were not using in the url, you could simple just add it as a hidden value on the page.

def post_add(request, thread_id):
    a_form = PostForm(request.POST)

    if(a_form.is_valid()):
        post = a_form.save(commit=False)
        post.thread_id = thread_id
        post.save() 

In the post_add view which accepts the thread_id and passes it to the method, we create the form based on the post data. validate it and save it. By adding the commit=False we stop the Form from saving the data to the database. and we return a model object. Which we can then pass along the thread_id and save it.

Their are a few things that could be added to the view for extra checking. We could do a lookup for a thread id and make sure its there, and do checks for request.method, or even add in handling for ajax requests. The example view isn’t complete and is only there as a quick sample of how to handle these types of forms.