Doing things with Django forms

Forms are one of the best features of Django. (After models, admin, url routing etc ). Here is a quick tutorial describing how to do things with Django forms.

1. Basic form

Prob. You want to show a form, validate it and display it.

Ans. Create a simple form.

class UserForm(forms.Form):
    username = forms.CharField()
    joined_on = forms.DateField()

This wil take care that the form is displayed with two text field, and a value for them are filled in, and the second field has correct formatting for a date.

2.

Prob. Create a form which has values populated depending upon a runtime value, eg you might want to show only the values corresponding to the current subdomain.

Ans. Create a form with an custom __init__.

class UserForm(forms.Form):
    username = forms.CharField()
    plan = forms.ModelChoiceField(queryset = Plan.objects.none())

    def __init__(self, subdomain, *args, **kwargs):
        self.default_username = default_username
        super(UserForm, self).__init__(*args, **kwargs)
        self.fields['plan'].queryset = Plan.objects.filter(subdomain = subdomain)

Here in the __init__ we are overriding the default queryset on field plan. Any of the attributes can similarly be overridden.

However the self.fields is populated only after super(UserForm, self).__init__(*args, **kwargs) is called.

3.

Prob. The same form is used in multiple views, and handles the cleaned_data similarly.

Ans. Create a form with a custom .save()

class UserForm(forms.Form):
    username = forms.CharField()

    def save(self):
        data = self.cleaned_data
        user = User.objects.create(username = data['username'])
        #create a profile
        UserProfile.objects.create(user = user, ...some more data...)

You could have called this method anything, but this is generally called save, to maintain similarity with ModelForm

4.

Prob. You need to create a form with fields which have custom validations.

Ans. Create a form with custom clean_fieldname

class UserForm(forms.Form):
    username = forms.CharField()

    def clean_username(self):
        data = self.cleaned_data
        try:
            User.objects.get(username = data['username'])
        except User.DoesNotExist:
            return data['username']
        raise forms.ValidationError('This username is already taken.')

Here we can validate that the usernames are not repeated.

5.

Prob. You want to create a field which has cross field validations.

Ans. Create a field with a .clean

class UserForm(forms.Form):
    username = forms.CharField()

    password1 = forms.PasswordField()
    password2 = forms.PasswordField()

    def clean(self):
        data = self.cleaned_data
        if "password1" in data and "password2" in data and data["password1"] != data["password2"]:
            raise forms.ValudationError("Passwords must be same")

6.

Problem.

You need a form the fields of which depends on some value in the database. Eg. The field to be shown are customisable per user.

Create a form dynamically

def get_user_form_for_user(user):
        class UserForm(forms.Form):
            username = forms.CharField()
            fields = user.get_profile().all_field()
            #Use field to find what to show.

This post provides much more details

7.

Prob. You need to create a Html form which writes to multiple Django models.

Ans. Django forms are not a one to one mapping to Html forms.

#in forms.py
class UserForm(forms.ModelForm):
    class Meta:
        model = User
        fields = ["username", "email"]

class UserProfileForm(forms.ModelForm):
    class Meta:
        model = UserProfile

#in views.py
def add_user(request):
    ...
    if request.method == "POST":
        uform = UserForm(data = request.POST)
        pform = UserProfileForm(data = request.POST)
        if uform.is_valid() and pform.is_valid():
            user = uform.save()
            profile = pform.save(commit = False)
            profile.user = user
            profile.save()
            ....
    ...

#in template
<form method="post">
    {{ uform.as_p }}
    {{ pform.as_p }}
    <input type="submit" ...>
</form>

8.

Prob. You want to use multiple forms of same type on one page.

Ans.

a. If you want a datagrid style ui, use formset.

from django.forms.formsets import formset_factory
forms = formset_factory(UserForm, extra = 4)
#

Formsets are described much more comprehensively here

b. If you do not need a datagrid style ui, use prefix argument to forms.

Eg. you have a survey app, and you want a page with all questions from that survey displayed.

#IN views.py
def survey(request, survey_slug)
    ...
    questions = survey.questions.all()
    question_forms = []
    for question in questions:
        qform = QuestionForm(question=question, prefix = question.slug)
        question_forms.append(qform)
        ...
    if request.method == "POST":
        for question in questions:
            qform = QuestionForm(question=question, prefix = question.slug, data = request.POST)
        #Validate and do save action
        ...
    ...

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