Django using a ManytoMany as a Foreign Key - django

Question

I'm trying to use a many to many relation as a foreign key for another table, but I'm not sure if this is possible.Consider the following model:
from django.db import models
class Attribute(models.Model):
name=models.CharField(max_length=30)
unit=models.CharField(max_length=30)
class Protocol(models.Model):
name=models.CharField(max_length=30)
attributes=models.ManyToManyField(Attribute,db_table='protocol_attribute')
class Device(models.Model):
serial_number = models.CharField(max_length=30)
id_protocol=models.ForeignKey(Protocol)
class DeviceValues(models.Model):
id_device=models.ForeignKey(Device)
id_protocol_attribute=models.ForeignKey(Protocol.attributes)
value=models.IntegerField()
When I try to validate this model I receive the following error:
First parameter to ForeignKey must be either a model, a model name, or the string 'self'
I know I am not using a model or a model name as the first parameter, but how can I reference as a foreign key the join table that Django will automatically create that handles the mapping between Protocol and Attributes?
A possible workaround could be to add the protocol as a foreign key to the Attribute class and then use the attribute as a foreing key to the DeviceValue class. However, I don't want to do that because I will have protocols that will be sharing attributes. I would prefer to make the join table rather that repeating attributes for each protocol.
Any ideas will be more than welcome....thanks!!!
Alejandro

Answer 1

Using an intermediate table, I solved it. The model ended up like this.
from django.db import models
class Attribute(models.Model):
name=models.CharField(max_length=30)
unit=models.CharField(max_length=30)
class Protocol(models.Model):
name=models.CharField(max_length=30)
attributes=models.ManyToManyField(Attribute,through='DeviceValue')
class Device(models.Model):
serial_number = models.CharField(max_length=30)
id_protocol=models.ForeignKey(Protocol)
class DeviceValue(models.Model):
id_name=models.ForeignKey(Protocol)
id_attribute=models.ForeignKey(Attribute)
id_device=models.ForeignKey(Device)
value=models.IntegerField()
One thing worth mention is that when using sqlall the UNIQUE constraint for the three foreign keys of the DeviceValue tablue was not generated, so I just added it before creating the database.