ManyToMany and "set" function

Hi,

i have a model with a m2m:

class PiattoPersonalizzato(models.Model):

    descrizione = models.CharField(max_length=80)

    categoria = models.ForeignKey(Categoria)

    numero = models.IntegerField(default=1)

    ingredienti = models.ManyToManyField(Ingrediente,default=None,null=True,blank=True)

    prezzo = models.DecimalField(max_digits=15, decimal_places=2, default=0)

    def __unicode__(self):

        return self.descrizione

    class Meta:

        verbose_name_plural = "Piatti Personalizzati"

I need to use the difference function, so 

piattoPersonalizzato = [... get the object model ... ]

piatto = [... get the object model ... ]

[...]

                ingredientiPiattoComanda = piatto.ingredienti.all()

                ingredientiPiattoPersonalizzato = piattoPersonalizzato.ingredienti.all()

                intersezione = set(ingredientiPiattoComanda).difference(set(ingredientiPiattoPersonalizzato))

                if not intersezione:

   [...]

It is possible do it using django database function? Do it cost a lot of memory, i want to do it using database...

regards,

Vanni

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