title: Filter django queryset including a specific object in manytomany field
body: <p>Here is my models,<br></p>
  
  <pre><code>class Age(models.Model):
      layer = models.CharField(max_length=50)
      order = models.PositiveIntegerField()
      ...
  class Course(models.Model):
      target_age = models.ManyToManyField('Age')
      ...
  </code></pre>
  
  <p>How can I get a course queryset including a specific age?<br>
  views:<br></p>
  
  <pre><code>request_courses = Course.objects.filter(target_age ...
  </code></pre>