admin.py

class AsignaturaNMEAdmin(admin.ModelAdmin):
    model = AsignaturaNME
    list_display = get_fieldnames(model)
    exclude = ['slug']
    search_fields = ['nombre', 'CampoFormativoNME', 'grado']

    class CustomModelChoiceField(forms.ModelChoiceField):
         def label_from_instance(self, obj):
             return "%s - %s - %s" % (obj.nombre, obj.ciclo.nivel, obj.ciclo)

    def formfield_for_foreignkey(self, db_field, request, **kwargs):
        if db_field.name == 'grado':
            return self.CustomModelChoiceField(
                queryset=Grado.objects.validados().order_by(
                    'ciclo__nivel',
                    'ciclo',
                    'nombre'
                ).exclude(ciclo__nivel=4)
            )

        return super(AsignaturaNMEAdmin, self).formfield_for_foreignkey(
            db_field, 
            request, 
            **kwargs
        )