Can not add a group to a custom auth user

[Ikaros andi]

I want create a user with custom group in django admin. so I write below 
code:

from django.contrib.auth.models import User as AuthUserfrom 
django.contrib.auth.models import Group
# These groups have already been created while project start.    class 
TestGroup(object):
    Admin = 'Admin'
    Merchant = 'Merchant'
    User = 'User'
class Merchant(AuthUser):

    def save(self, **kwargs):
        super(Merchant, self).save(**kwargs)

        for group in Group.objects.all():
        print group.name

        # way 1
        if not self.groups.filter(name=TestGroup.Merchant).exists():
            print self.groups.filter(name=TestGroup.Merchant).exists()
            g = Group.objects.get(name=TestGroup.Merchant)
            g.user_set.add(self)
            print self.groups.filter(name=TestGroup.Merchant).exists()

        # way 2
        if not self.groups.filter(name=TestGroup.Merchant).exists():
            g = Group.objects.get(name=TestGroup.Merchant)
            self.groups.add(g)


        # way 3
        if not self.groups.filter(name=TestGroup.Merchant).exists():
            g = Group.objects.get(name=TestGroup.Merchant)
            self.groups.add(g)
            self.save()

I have tried three ways to add a group to a user.But none of them could 
work.

*For Test:*

You can test by following this steps:

   1. 
   
   create a group named 'Merchant' at django admin
   2. 
   
   add my code (add print in way 1 to test), syncdb and so on.
   3. 
   
   create a Merchant at django admin. you can see log:
   
   u'Merchant'
   False
   True
   4. 
   
   enter the merchant you just created, you can see, the group merchant is 
   not selected(means this user do not beyond to this group).
   5. 
   
   click save again, you would still see
   
   u'Merchant'
   False
   True
   
add group to merchant fail, very strange.

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