Modeling accounts for different types of users in Django

Say that you have an application where different kind of users can sign: Firms, Lawyers, and Clients. A Firm has many lawyers; a lawyer has many clients. The views for a firm user are, of course, different from the views of a lawyer user; the two are different from the client user.

How would you model the three different users? I can think the following approach:

Three different models with a ForeignKey to User, each with their own fields, e.g.:

class Firm(models.Model):
 user = models.ForeignKey(User)
class Lawyer(models.Model):
 user = models.ForeignKey(User)
 specialty = models.CharField(max_length=100)
class Client(models.Model)
 user = modelsForeignKey(User)

Now you can create, for instance, consultations as a separate model using two ForeignKeys: to Lawyer and to Client; you can also add resources to a consultation (like documents, or stuff like that) by creating a model Resource with a ForeignKey to Consultation.

This approach makes it difficult to distinguish among users: how do you know whether a user is a Firm, for instance - you need to query the database several times or assign a Profile to the generic User object.

You could also add only a Profile to the User and include a Role, and then you channel the views and authentication based on user.get_profile().role.

How would you deal with this problem?