Django: add empty choice to СhoiceField for ModelForm

gistfile1.py

from django.db import models

class Project(models.Model):
    name = models.CharField(max_length=140)
    parent = models.ForeignKey('self', blank=True, null=True)

class ProjectForm(ModelForm):
    def __init__(self, *args, **kwargs):
        super(ProjectForm, self).__init__(*args, **kwargs)
        ordered = []
        // ... making ordered list ...
        self.fields['parent'].choices = ordered
self.fields['parent'].choices.insert(0, ('', u'------'))

# You can import blanc field from:
# from django.db.models.fields import BLANK_CHOICE_DASH