title: Django admin. Limit choices of many to many field
body: <p>I have a Many to Many field. I'd like to limit the choices the admin shows in its M2M widget.</p>
  
  <p>I have a model like this:</p>
  
  <pre><code>class A(models.Model):
      b_field = models.ManyToManyField(B)
  
  class B(models.Model):
      available = models.BooleanField()
  </code></pre>
  
  <p>How do I limit the B objects shown in the widget only to those who have <code>available = True</code>?</p>