Prefill wagtailforms with URL parameters

models.py

from __future__ import absolute_import, unicode_literals

import django
import django.forms
import wagtail.wagtailforms.models as form_models
from django.db import models
from django.shortcuts import render
from django.utils.translation import ugettext_lazy as _
from modelcluster.fields import ParentalKey
from wagtail.wagtailadmin.edit_handlers import (
    FieldPanel, FieldRowPanel,
    InlinePanel, MultiFieldPanel
)
from wagtail.wagtailcore.fields import RichTextField
from wagtail.wagtailcore.models import Page
from wagtail.wagtailforms.forms import FormBuilder
from wagtail.wagtailforms.models import AbstractEmailForm, AbstractFormField

FORM_FIELD_CHOICES = form_models.FORM_FIELD_CHOICES + (
    ('hidden', _('Hidden')),
)


class FormField(AbstractFormField):
    field_type = models.CharField(verbose_name=_('field type'), max_length=16, choices=FORM_FIELD_CHOICES)
    page = ParentalKey('FormPage', related_name='form_fields')


class CmsFormBuilder(FormBuilder):
    def create_hidden_field(self, field, options):
        options['max_length'] = 255
        return django.forms.CharField(widget=django.forms.HiddenInput, **options)

    FormBuilder.FIELD_TYPES['hidden'] = create_hidden_field


class FormPage(AbstractEmailForm):
    form_builder = CmsFormBuilder
    intro = RichTextField(blank=True)
    outro = RichTextField(blank=True)
    thank_you_text = RichTextField(blank=True)

    content_panels = AbstractEmailForm.content_panels + [
        FieldPanel('intro', classname="full"),
        InlinePanel('form_fields', label="Form fields"),
        FieldPanel('outro', classname="full"),
        FieldPanel('thank_you_text', classname="full"),
        MultiFieldPanel([
            FieldRowPanel([
                FieldPanel('from_address', classname="col6"),
                FieldPanel('to_address', classname="col6"),
            ]),
            FieldPanel('subject'),
        ], "Email"),
    ]

    def serve(self, request, *args, **kwargs):
        if request.method == 'POST':
            form = self.get_form(request.POST, page=self, user=request.user)

            if form.is_valid():
                self.process_form_submission(form)

                # render the landing_page
                # TODO: It is much better to redirect to it
                return render(
                    request,
                    self.get_landing_page_template(request),
                    self.get_context(request)
                )
        else:
            form = self.get_form(page=self, user=request.user)
            for key in request.GET:
                try:
                    form.fields[key].initial = request.GET[key]
                except KeyError:
                    # Ignore unexpected parameters
                    pass

        context = self.get_context(request)
        context['form'] = form
        return render(
            request,
            self.get_template(request),
            context
        )