Tag "complex"

Complex Formsets, Redux

Background ========== Two years ago, Malcolm Tredinnick put up an excellent post about doing dynamic Django forms. There have been several excellent write-ups on it since - Google is your friend. One year ago, I attempted to make a dynamic Formset - see [this snippet](http://www.djangosnippets.org/snippets/1290/). Malcolm posted a cleaner solution two weeks later, and I liked his solution better. Some time after that happened, his site tanked. I'm re-posting my snippet using his technique, so that everyone can see how it is done. Credit to goes to Malcolm - I'm just the messenger. If his site ever comes back up , check out his complex formset post [here](http://www.pointy-stick.com/blog/2009/01/23/advanced-formset-usage-django/). I'll use Malcolm's example code, with as few changes as possible to use a formset. The models and form don't change, and the template is almost identical. I won't reproduce all of Malcolm's code - I'm just show the form setup. There are no models here - you'll have to use your imagination for Quiz, Question, and Answer models. He did some fancy validation as well - I'm not going to here. Problem ======= Build a formset based on dynamically created forms. Solution ======== The core idea in this code is found in the `_construct_form` method. Typically, this is where a formset makes new forms - it handles indexing them so that everything is nice and unique. We can take advantage of this by overriding the method and inserting a `kwarg` that will be passed on to our form class, then calling the parent `_contruct_form` method to let it finish doing everything else for us. This is what Malcolm, a core Django developer, knows about, and I, a random Django user, typically do not. Code ==== This pattern greatly simplifies building formsets dynamically, and it really only requires a few bits of knowledge. 1. If we `pop()` special arguments out of `kwargs` dictionaries, we then can pass the remaining `kwargs` along to parent methods and let them do the rest of the setup for us. See code tricks #1 and #3. 2. If we have a form and need to add dynamic fields that we didn't declare the usual way, we can just add them to the `self.fields` dictionary. See code trick #2. 3. If we need to add forms dynamically to a formset, we can use the `self.extra` variable to specify how many we want, based on the length of a custom queryset. See code trick #4. 4. If we want to pass some special arguments to a form that will be part of a formset when it is constructed, we can add them to the `kwargs` dict in `_construct_form`, taking advantage of the `index` variable to track which object from our queryset we wanted. See code trick #5.