filter_horizontal not work

I am a new Django User.
I am doing an aplication web.
My problem is the next:
When I put (admin.py)
class FarmAdmin(admin.ModelAdmin):
filter_horizontal = ('server',)
===models.py====
class Farm(models.Model):
farm_id = models.IntegerField(primary_key=True)
name = models.CharField(max_length=50)
server = models.ManyToManyField(Server)

It's work good, but if i save it, I have differents problems with DB
fields. However, if I put server = models.ManyToManyField(Server,
through="FarmServer")

It's dont work!! But I haven't problem. I cant see the widget. I only
see farm_id and name fields.

class FarmServer(models.Model):
farm = models.ForeignKey(Farm, primary_key=True)
server = models.ForeignKey(Server,primary_key=True)
class Meta:
db_table = u'farms_servers'

what can I do???
Thanks and sorry for my english.

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