title: Django Rest-Framework error using Post method with manytomany relationship
body: <p>I have the next code:</p>
  
  <p>Models:</p>
  
  <pre><code>class Producto(models.Model):
  
      def __unicode__(self):
          return(self.nombre)
  
      nombre = models.CharField(max_length=50)
      descripcion = models.TextField(max_length=140)
      color = models.CharField(max_length=15)
      talla = models.CharField(max_length=10)
      precio = models.IntegerField()
      cantidad = models.IntegerField()
      create_date = models.DateTimeField("Fecha de Creacion",auto_now=True)
  
      def precio_display(self):
  
          return "Gs. "+ format(self.precio, "8,d")
  
      precio_display.short_description = 'Precio'
  
  
  class Venta(models.Model):
  
      def __unicode__(self):
          return "id: {0}, {1:%H:%M - %d/%m/%Y}".format(self.id,  self.fecha)
  
      fecha = models.DateTimeField(auto_now=True)
      # Relacion muchos a muchos por medio de la tabla Detalle
      productos = models.ManyToManyField('Producto', through="Detalle",  related_name="productos")
      total = models.IntegerField()
      credencial = models.CharField(max_length=200)
  
  
  class Detalle(models.Model):
  
      producto = models.ForeignKey(Producto)
      venta = models.ForeignKey(Venta)
      cant = models.IntegerField()**`strong text`**
      precioVenta = models.IntegerField()
  
      def __unicode__(self):
          return "Se vendio {0} de {1} en la venta {2}".format(self.cantidad,self.producto, self.venta.id)
  </code></pre>
  
  <p>Serializers:</p>
  
  <pre><code>class ProductoModelSerializer(ModelSerializer):
  
      class Meta:
          model = Producto
          fields = ("id", "nombre", "descripcion", "color", "talla",
                    "precio", "cantidad")
  
  
  class DetalleSerializer(HyperlinkedModelSerializer):
  
      id = ReadOnlyField(source='producto.id')
      nombre = ReadOnlyField(source='producto.nombre')
  
      class Meta:
          model = Detalle
  
          fields = ('id', 'nombre', 'cant', 'precioVenta')
  
  
  class VentaModelSerializer(ModelSerializer):
      productos = DetalleSerializer(source='detalle_set', many=True)
  
      class Meta:
          model = Venta
          fields = ("id", "productos", "total", "credencial")
  </code></pre>
  
  <p>and when i use Post method i get the next error:</p>
  
  <pre><code>TypeError at /stock/rest/ventas/
  'Detalle' instance expected, got OrderedDict([(u'cant', 25), (u'precioVenta', 25000)])
  Request Method: POST
  Request URL:    http://127.0.0.1:8000/stock/rest/ventas/
  Django Version: 1.7.5
  Exception Type: TypeError
  Exception Value:    
  'Detalle' instance expected, got OrderedDict([(u'cant', 25), (u'precioVenta', 25000)])
  </code></pre>
  
  <p>i dont know why. The get method work perfectly when i use this with a web forntend done with angular.js. But the POST, PATCH and PUT method get me the same error.</p>
  
  <p>PD: Sorry for my bad english.</p>