[Python] How to change/filter option list in TreeNodeChoiceField in a TabularInline

How to change the option/choice list of conto field, based on value gruppo_id?

conto is a ForeignKey to a branch of a tree described as model.ContoCOGE(MPTTModel). For each gruppo there is a tree, which develops to a certain depth. How to filter among all the trees and, in TreeNodeChoiceField in a TablularInline, to list only that with a group_id?

In debug, formfield_for_foreignkey(self, db_field, request, **kwargs) overriden returns the correct tree. But the browser renders the page with the select field with all the trees.

#model.py
from mptt.models import MPTTModel, TreeForeignKey
from multigroup.models import Gruppo

class ContoCOGE (MPTTModel):
gruppo = models.ForeignKey(Gruppo, on_delete=models.CASCADE,related_name='contoCOCGE_gruppo')
parent = TreeForeignKey('self', on_delete=models.CASCADE, null=True, blank=True, related_name='children')

class PncGen (models.Model):
gruppo = models.ForeignKey(Gruppo, on_delete=models.CASCADE)
#other stuff

class PncRighe (models.Model):
gruppo = models.ForeignKey(Gruppo, on_delete=models.CASCADE)
idPnc = models.ForeignKey(PncGen,on_delete=models.CASCADE) #puntatore alla testata
conto = models.ForeignKey(ContoCOGE,on_delete=models.CASCADE,blank=True, null=True)
#other stuff


#admin.py
from django_mptt_admin.admin import DjangoMpttAdmin

class PncRigheInline(admin.TabularInline):
model = PncRighe
autocomplete_fields = ['partitarioRiga','conto']
extra = 1

def filter_tree_queryset(self, queryset,request):
qs = queryset.filter(gruppo_id=int(request.session['gruppo_utente']))
return qs

def get_changeform_initial_data(self, request):
return {'gruppo': int(request.session['gruppo_utente'])}

def formfield_for_foreignkey(self, db_field, request, **kwargs):
if db_field.name == 'gruppo' or db_field.name == 'idRiga' or db_field.name == 'idPnc':
kwargs['widget'] = forms.HiddenInput()

if db_field.name == 'conto':

db = kwargs.get("using")
qs = self.get_field_queryset(db, db_field, request)
kwargs['queryset']=qs.filter(gruppo_id = request.session['gruppo_utente'] )

@admin.register(PncGen)
class PncGenAdmin(admin.ModelAdmin):
#...
inlines = (PncRigheInline,)

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