When using ModelAdmin.save_model, an existing instance is inserted again, instead of being updated

[anonymous]

When I change an existing Magasin instance from the admin, I get an IntegrityError, eg:

"Duplicate entry '220' for key 'PRIMARY'"

meaning that in Model.save, the instance is duplicated, instead of being updated.

When I do it programmatically, it works as expected, and the instance is updated:

m = Magasin()
m.nom="test"
m.region_id=3
m.save()

m.nom="test_modified"
m.save() # no error

Bug?

models.py

class Magasin(models.Model):
    nom = models.CharField(max_length=200)
    region = models.ForeignKey(Region, blank=True, null=True)

    def save(self, *args, **kwargs):
        if self.region_id is None:
            self.region_id = args[0]
        super(Magasin, self).save(*args, **kwargs)

admin.py

class MagasinAdmin(admin.ModelAdmin):

    def save_model(self, request, obj, form, change):
        # more stuff (using request object)
        obj.save()