title: Django models filter by foreignkey
body: <p>I'm having some trouble in filtering objects from a set of models. Here is the problem:</p>
  
  <p>I have 3 classes:</p>
  
  <pre><code>class Autor(models.Model):    
      nome = models.CharField(max_length=50)
      slug = models.SlugField(max_length=50, blank=True, unique=True)
      foto = models.ImageField(upload_to='autores/', null=True, blank=True)
      ...
  
  class CategoriaRecolha(models.Model):
      categoria = models.CharField(max_length=30)
      descricao = models.TextField()
      slug = models.SlugField(max_length=30, blank=True, unique=True)
      ...
  
  class Recolha(models.Model):    
      titulo = models.CharField(max_length=100)
      slug = models.SlugField(max_length=100, blank=True, unique=True)
      descricao = models.TextField()
      ficha_tec = models.TextField()
      categoria = models.ForeignKey(CategoriaRecolha)
      autor = models.ForeignKey(Autor)
      ....
  </code></pre>
  
  <p>What I'm trying to retrieve is the fields of the class Autor, in which the field categoria of the class Recolha is equal to a specific value.</p>
  
  <p>In a more simple way, I need to get the all the autor that have participated in a specific categoria.</p>
  
  <p>Thanks</p>