title: Django: get subset of a model with at least one related model
body: <pre><code>class Category(models.Model):
      # fields
  
  class Product(models.Model):
       category = models.ForeignKey(Category)
       # fields
  </code></pre>
  
  <p>Assuming that not all the categories have at least a product,</p>
  
  <p><strong>how i can get all the categories that have at least one product associated</strong>?</p>
  
  <p>Is there a way to do this with Django querysets?</p>