title: 'ValuesListQuerySet' object has no attribute 'group_by'
body: <p>I am trying to display sizes and colors of a store product according to a store:</p>
  
  <pre><code>store1: sizes=S,M,L colors=Red,blue
  
  store2: sizes=L,Xl  colors=yellow,green
  </code></pre>
  
  <p>I tried to use <code>values_list</code> along with <code>group_by</code> but it throws the error</p>
  
  <blockquote>
    <p>'ValuesListQuerySet' object has no attribute 'group_by'</p>
  </blockquote>
  
  <p>It seems like <code>group_by</code> is not a part of django. What can I do to group my query as above?</p>
  
  <p><strong>models.py</strong></p>
  
  <p>A store contains a product which can be of different sizes and colors. Similarly a product can be available in more than one store.</p>
  
  <pre><code>class StoreProduct(models.Model):
      product = models.ForeignKey('products.Product')
      category = models.ForeignKey('products.Category')
      store = models.ForeignKey('Store')
      price = models.IntegerField(null=True , blank=True)
      quantity = models.IntegerField(null=True , blank=True)
      sizes = models.CharField(max_length=50 , null=True , blank=True)
      colors = models.CharField(max_length=50 , null=True , blank=True)
  
      def __unicode__(self):
          return self.product.title
  
  
  class Store(models.Model):
      user = models.OneToOneField(settings.AUTH_USER_MODEL , null=True , blank=True)
      StoreName = models.CharField(max_length=50 , default = '')
      items = models.ManyToManyField('products.Product', through=StoreProduct)
      brand = models.ForeignKey('products.Brand' , null=True , blank=True)
      City = models.CharField(max_length=50 , default = '')
      Area = models.CharField(max_length=50 , default = '')
      Address = models.TextField(max_length=500)
      MallName = models.CharField(max_length=50 , null=True , blank=True)
      slug = models.SlugField(blank=True ,unique=True)
  </code></pre>
  
  <p><strong>views.py</strong></p>
  
  <pre><code> StoreProduct.objects.filter(product=productobj) \
     .values_list('sizes', flat=True) \
     .group_by('store')
  </code></pre>