Unable to save m2m relation modelform - django

Question

I'm struck on saving m2m relation.
models.py
class BasicTag(models.Model):
name = models.CharField(max_length=150, verbose_name="Tag Name")
image_class = models.CharField(max_length=30, blank=True)
color = models.CharField(max_length=10, blank=True)
def __unicode__(self):
return self.name
class ExtendedTag(models.Model):
parent = models.ManyToManyField(BasicTag, blank=True,
related_name="parent")
category = models.ManyToManyField(BasicTag, blank=True,
related_name="category")
def __unicode__(self):
return self._id
class CombineTag(BasicTag, ExtendedTag):
"""
"""
forms.py
class CombineTagForm(forms.ModelForm):
class Meta:
model = CombineTag
Now when I initialize form as
>form = CombineTagForm({"name":"shyam"})#this don't have any parent i.e. root tag
>form.is_valid()
True
>instance = form.save(commit = False)
>instance.save()
>form.save() #return errors
#error
ProgrammingError: column tag_extendedtag_parent.basictag_id does not exist
LINE 1: DELETE FROM "tag_extendedtag_parent" WHERE "tag_extendedtag_...
>form.save_m2m() #return errors ... struck here
So How should I need to save m2m field modelform. I had follow official doc which say that
If your model has a many-to-many relation and you specify commit=False when you save a form, Django cannot immediately save the form data for the many-to-many relation. This is because it isn’t possible to save many-to-many data for an instance until the instance exists in the database.
But I couldn't able to figure out what I am missing here.