Set parent in a ModelForm in Google App Engine

I want to create an Entity Group relationship in an Entity that is being created through a ModelForm.

How do I pass the parent instance and set the parent= attribute in the ModelForm?

I'll be interested to see if you get any good solutions to this problem. My own solution, which is far from elegant, is to do this:

book = models.Book(title='Foo')
chapter = models.Chapter(parent=book, title='dummy')
form = forms.ChapterForm(request.POST, request.FILES, instance=chapter)

Basically, I first create a dummy object (chapter in this case) with the correct parent relationship and then pass that as the instance argument to the form's constructor. The form will overwrite the throwaway data I used to create the dummy object with the data given in the request. At the end, to get the real child object, I do something like this:

if form.is_valid():
    chapter = form.save()
    # Now chapter.parent() == book

I subclassed djangoforms.ModelForm and added a create method*:

class ModelForm(djangoforms.ModelForm):
  """Django ModelForm class which uses our implementation of BoundField.
  """

  def create(self, commit=True, key_name=None, parent=None):
    """Save this form's cleaned data into a new model instance.

    Args:
      commit: optional bool, default True; if true, the model instance
        is also saved to the datastore.
      key_name: the key_name of the new model instance, default None
      parent: the parent of the new model instance, default None

    Returns:
      The model instance created by this call.
    Raises:
      ValueError if the data couldn't be validated.
    """
    if not self.is_bound:
      raise ValueError('Cannot save an unbound form')
    opts = self._meta
    instance = self.instance
    if self.instance:
      raise ValueError('Cannot create a saved form')
    if self.errors:
      raise ValueError("The %s could not be created because the data didn't "
                       'validate.' % opts.model.kind())
    cleaned_data = self._cleaned_data()
    converted_data = {}
    for name, prop in opts.model.properties().iteritems():
      value = cleaned_data.get(name)
      if value is not None:
        converted_data[name] = prop.make_value_from_form(value)
    try:
      instance = opts.model(key_name=key_name, parent=parent, **converted_data)
      self.instance = instance
    except db.BadValueError, err:
      raise ValueError('The %s could not be created (%s)' %
                       (opts.model.kind(), err))
    if commit:
      instance.put()
    return instance

Usage is simple:

book = models.Book(title='Foo')
form = forms.ChapterForm(request.POST)
chapter = form.create(parent=book)

Note that I did not copy/paste the code that lets you specify the key_name in request.POST, instead I have it passed as an argument to create.

* Code is modified from the save method of the original modelform in google.appengine.ext.db.djangoforms.