gistfile1.py

class EditTaskForm(forms.ModelForm):
    class Meta:
        model = Task
        fields = ('name', 'period', 'responsible')
        exclude = ('at_project', )

    def __init__(self, *args, **kwargs):
        super(EditTaskForm, self).__init__(*args, **kwargs)
        self.project = kwargs.get('project')

    def save(self, **kwargs):
        task = super(EditTaskForm, self).save()
        task.at_project = self.project
        task.save()