title: Generic foreign key in Django showing error must be instance of Content Type
body: <p>I have a following abstract Class </p>
  
  <pre><code>class Manufacturer(models.Model):
      company=models.CharField(max_length=255)
  
      class Meta:
          abstract = True
  </code></pre>
  
  <p>Now 2 classes inherit from above:-</p>
  
  <pre><code>class Car(Manufacturer):
      name = models.CharField(max_length=128)
  
  class Bike(Manufacturer):
      name = models.CharField(max_length=128)
  </code></pre>
  
  <p>Now I want to link them with features, so I create following classes:-</p>
  
  <pre><code>class Feature(models.Model):
      name= models.CharField(max_length=255)
      limit=models.Q(model = 'car') | models.Q(model = 'bike')
      features = models.ManyToManyField(ContentType, through='Mapping',limit_choices_to=limit)
  
  class Mapping(models.Model):
      category=models.ForeignKey(Category, null=True, blank=True)
      limit=models.Q(model = 'car') | models.Q(model = 'bike')
      content = models.ForeignKey(ContentType, on_delete=models.CASCADE,limit_choices_to=limit,default='')
      object_id = models.PositiveIntegerField(default=1)
      contentObject = GenericForeignKey('content', 'object_id')
  
      class Meta:
          unique_together = (('category', 'content','object_id'),)
          db_table = 'wl_categorycars'
  </code></pre>
  
  <p>But When i try creating instances in shell commands I get an error while creating mapping instance</p>
  
  <blockquote>
    <p><strong>"Mapping.content" must be a "ContentType" instance.</strong></p>
  </blockquote>
  
  <pre><code>car1=Car(company="ducati",name="newcar")
  bike1=Bike(company="bike",name="newbike")
  
  cat1=Category(name="speed")
  
  mapping(category=cat1, content=car1)  # ---&gt; i get error at this point
  </code></pre>
  
  <p>How do I proceed with this?</p>