title: giving different related names for a foreign key for the children in django multitable inheritance
body: <p>I have these models:</p>
  <pre><code>class Theme(models.Model):
     name = models.charfield()
  class Product(models.Model):
      theme = models.ForeignKey(Theme)
  class PhysicalProduct(Product)
  class Course(Product):
      ........
  class Book(Product):
     ........
  </code></pre>
  <p>and what I want to do is to fetch only the courses and the books related to a specific theme, with <code>prefetch_related</code> method but the problem is that I don't think there's a way to do that but to add the theme field to each child of the <code>Product</code> model and give each one a specifically related name:</p>
  <pre><code>class Theme(models.Model):
     name = models.charfield()
  class Product(models.Model):
      .......
  class PhysicalProduct(Product):
     theme = models.ForeignKey(Theme, related_name = 'physical_products')
  class Course(Product):
      theme = models.ForeignKey(Theme, related_name = 'courses')
      ........
  class Book(Product):
     theme = models.ForeignKey(Theme, realted_name = 'books')
     ........
  </code></pre>
  <p>is there a way that I can keep the theme field in the <code>Product</code> model and be able to differentiate between specific types of products that I only wanna fetch with prefetch_related</p>