title: add value in table manytomany django
body: <p>hi i try to add same value in a table many to many in django but i don't know how can i do it</p>
  
  <p>this is my modles:</p>
  
  <pre><code>class Raza(models.Model):
       Nombre = models.CharField(max_length=50)
       Origen = models.CharField(max_length=45)
       Altura = models.CharField(max_length=10)
       Peso = models.CharField(max_length=10)
       Esperanza_vida = models.CharField(max_length=10)
       Actividad_fisica = models.CharField(max_length=45)
       Recomendaciones = models.CharField(max_length=500)
       Clasificacion_FCI = models.ForeignKey(Clasificacion_FCI,null=True,blank=True,on_delete=models.CASCADE)
       Tipo_pelo = models.ManyToManyField(Tipo_pelo,blank=True)
       Caracteristicas_fisicas = models.ManyToManyField(Caracteristicas_fisicas,blank=True)
       Caracter = models.ManyToManyField(Caracter,blank=True)
       Ideal = models.ManyToManyField(Ideal,blank=True)
       Tamanio = models.ForeignKey(Tamanio,null=True,blank=True,on_delete=models.CASCADE)
       User = models.ManyToManyField(User,blank=True)
  </code></pre>
  
  <p>and the User model are the default model that Django give me</p>
  
  <p>i tried someone like that </p>
  
  <pre><code>class AgregarFav(views.APIView):
      def post(self, request, *args, **kwargs):
          idUsario= request.POST.get('isUsuario')
          idPerro = request.POST.get('idPerro')
          raza = Raza.User.add(idPerro,idUsario)
          raza.save()
          return HttpResponse(idUsario)
  </code></pre>
  
  <p>but i have the error 'ManyToManyDescriptor' object has no attribute 'add'</p>
  
  <p>i want do something like that</p>
  
  <pre><code>table user 
  id_usuario = 1
  name = "Juan"
  
  table raza
  id_raza = 1 
  name = "pitbull"
  
  table user_raza
  id_user_raza = 1
  id_user = 1
  id_raza = 1
  </code></pre>