Django MPTT duplicate key value violates unique constraint

I practice django-mptt and face problem

Here is my models.py

class Genre(MPTTModel):
    name = models.CharField(max_length=50, unique=True)
    parent = TreeForeignKey('self', null=True, blank=True, related_name='children', db_index=True)

I create some data :

• USA
  • NY
    • BuildingA
      • 3F
      • 4F
    • BuildingB

And now,I need to create 4F under BuildingB

floor3 = Genre.objects.create(name="3F", parent= buildingB)

but face problem :

IntegrityError: duplicate key value violates unique constraint "myapp_genre_name_key"
DETAIL:  Key (name)=(3F) already exists.  

How can I fix it??

In my understanding, you want your names to be unique across the building. To achieve that, remove the unique=True from the name field declaration and add a Meta option:

class Genre(MPTTModel):
    name = models.CharField(max_length=50)
    parent = TreeForeignKey('self', null=True, blank=True, related_name='children', db_index=True)

    class Meta:
        unique_together = (('name', 'parent', ), )