title: Django DRF - How can i retrive an object if i have only the foreign keys but not the pk with RetriveAPIView
body: <p>I have 2 models (one is a custom user model) with a relation ManyToMany through a third model. I have the pks of the two first models in the client side and i need to retrive the relationship model object with a RetriveAPIView. How can i do it? Here is the code i got:</p>
  <p>models.py</p>
  <pre><code>class Patient(models.Model):
      rut = models.CharField(max_length=50)
      userAccount = models.ManyToManyField('accounts.UserAccount', through='therapy.Therapy')
  
  class Therapy(models.Model):
      patient= models.ForeignKey(Patient, on_delete=models.CASCADE)
      userAccount = models.ForeignKey(UserAccount, on_delete=models.CASCADE)
      isActive = models.BooleanField(default=True)
  </code></pre>
  <p>views.py</p>
  <pre><code>class UpdateRetriveTherapyView(RetrieveUpdateAPIView):
      queryset = Therapy.objects.all()
      serializer_class = TherapySerializer
  </code></pre>
  <p>urls.py (project urls)</p>
  <pre><code>urlpatterns = [
      path('api/patient/', include('apps.patient.urls')),
      path('api/therapy/', include('apps.therapy.urls')),
      path('admin/', admin.site.urls),
      path('auth/', include('djoser.urls')),
      path('auth/', include('djoser.urls.jwt')),
  ]
  
  urlpatterns += [
      re_path(r'^.*', TemplateView.as_view(template_name='index.html'))
  ]
  
  urlpatterns += [
      path('api-auth/', include('rest_framework.urls'))
  ]
  </code></pre>
  <p>therapy.urls.py (the relationship model urls)</p>
  <pre><code>urlpatterns = [
      path('&lt;int:pk&gt;', UpdateTerapiaView.as_view()),
  ]
  </code></pre>
  <p>may be there is a way to pass fk as a parameter in the url but idk.</p>