Problem with intermediary table from legacy database

Under Django 1.5.1 I'm having a go at using ManyToManyField (see below models.py and admin.py files) referring to an Sqlite3 legacy db. 

I'm receiving the following fatal error message in admin

Exception Value:	'LibroOption.fields' can't include the ManyToManyField field  'autore' because 'autore' manually specifies a 'through' model

(I've also tried to let Django to automagically define the intermediary table eliminating from models.py the through option, and it works like a charm: that is, when as admin  I open the Libri model I see the autore field horizontally filtered.)

How can I solve this problem having to necessarily use this legacy db and its tables which are fed by a Filemaker procedure too?

Thanks from Rome

Vittorio

========================

models.py

from django.db import models

# Create your models here.

class Autore(models.Model):

   nome = models.CharField(db_index=True,max_length=50) 

   cognome = models.CharField(db_index=True,max_length=50)

   def __unicode__(self):

       return u"%s %s" % (self.nome, self.cognome)

   class Meta:

        verbose_name_plural = "Autori" 

        db_table="Autori"

        ordering=['cognome', 'nome']

...................

...................

class Libro(models.Model):

   titolo = models.CharField(db_index=True,max_length=200) 

   autore = models.ManyToManyField(Autore,through='LibriAutori')

   def __unicode__(self):

      return self.titolo

   class Meta:

        verbose_name_plural = "Libri"

        db_table="Libri"

        ordering=['titolo']

class LibriAutori(models.Model):

   libro = models.ForeignKey(Libro,db_column='libro')

   autori = models.ForeignKey(Autore,db_column='autori') 

   class Meta:

        verbose_name_plural = "LibriAutori"

   db_table='LibriAutori'

=====================================

admin.py

from django.contrib import admin

from biblio.models import *

from django import forms

#

class LibroOption(admin.ModelAdmin):

    list_display = ('titolo','autore')

    fields=(('titolo'),'autore')

    filter_horizontal = ['autore', ]

    order_by= ['titolo', ]

admin.site.register(Autore)

admin.site.register(Libro, LibroOption)

admin.site.register(LibriAutori)