Django autocategorize in m2m field

I have done a pre_save signal in my django/satchmo inherited model Product called JPiece and I have another model inheritance from satchmo Category called JewelCategory. The pre_save signal makes the JPiece objects get the category list and add those categories that fit the Jpiece description to the relation, that is done in the model, meaning if I manually do

p = Jpiece.objects.get(pk=3) p.save()

The categories are saved and added to the p.category m2m relation but If i save from the admin it does not do this...

How can I achieve this... to save from the admin a JPiece and to get the categories it belongs too...

Here are the models remember that they both have model inheritance from satchmo product and category classes.

class Pieza(Product):
    codacod = models.CharField(_("CODACOD"), max_length=20,
        help_text=_("Unique code of the piece. J prefix indicates silver piece, otherwise gold"))
    tipocod = models.ForeignKey(Tipo_Pieza, verbose_name=_("Piece Type"),
        help_text=_("TIPOCOD"))
    tipoenga = models.ForeignKey(Engaste, verbose_name=_("Setting"),
        help_text=_("TIPOENGA开发者_开发百科"))
    tipojoya = models.ForeignKey(Estilos, verbose_name=_("Styles"),
        help_text=_("TIPOJOYA"))
    modelo = models.CharField(_("Model"),max_length=8,
        help_text=_("Model No. of casting piece."),
        blank=True, null=True)



def autofill(self):
    #self.site = Site.objects.get(pk=1)
    self.precio = self.unit_price
    self.peso_de_piedra = self.stone_weigth
    self.cantidades_de_piedra = self.stones_amount
    self.for_eda = self.for_eda_pieza
    if not self.id:
        self.date_added = datetime.date.today()
        self.name = str(self.codacod)
        self.slug = slugify(self.codacod, instance=self)

    cats = []
    self.category.clear()

    for c in JewelCategory.objects.all():
        if not c.parent:
            if self.tipocod in c.tipocod_pieza.all():
                cats.append(c)
        else:
            if self.tipocod in c.tipocod_pieza.all() and self.tipojoya in c.estilo.all():
                cats.append(c)

    self.category.add(*cats)

def pieza_pre_save(sender, **kwargs):
    instance = kwargs['instance']
    instance.autofill()
#    import ipdb;ipdb.set_trace()

pre_save.connect(pieza_pre_save, sender=Pieza)

I know I can be vague with explanations sometimes of what I need so please feel free to ask anything Ill be sure to clarify ASAP since this is a client that needs this urgently.

Thank you all as always...

If you use pre_save, it's called before save(), meaning you can't define m2m relationships since the model doesn't have an ID.

Use post_save.

# this works because the ID does exist
p = Jpiece.objects.get(pk=3) 
p.save()

Update, check out the comment here: Django - How to save m2m data via post_save signal?

It looks like the culprit now is that with an admin form, there is a save_m2m() happening AFTER the post_save signal, which could be overwriting your data. Can you exclude the field from the form in your ModelAdmin?

# django.forms.models.py
if commit:
    # If we are committing, save the instance and the m2m data immediately.
    instance.save()
    save_m2m()