title: Django bulk_create causes duplicate entry integrity error
body: <p>I have the following model:</p>
  
  <pre><code>class GeneratedContent(models.Model):
      entity = models.ForeignKey('companies.Entity')
      source_url = models.URLField(max_length=255)
      title = models.CharField(max_length=255, blank=True, null=True)
      desc = models.TextField(blank=True, null=True)
      created_at = models.DateTimeField(auto_now_add = True)
      updated_at = models.DateTimeField(auto_now = True)
  
      def __str__(self):  
          return self.entity.name +' Content'
  </code></pre>
  
  <p>I am then processing some urls and then saving a bulk number of these objects like this:</p>
  
  <pre><code>gen_content_list = []
          for e in entities:
              entity_status = get_tweets(e.twitter_handle())
  
              try:
                  stat_url = re.search("(?P&lt;url&gt;https?://[^\s]+)", entity_status).group("url")
                  gen_content = GeneratedContent.objects.create(
                      entity=e,
                      desc=entity_status,
                      source_url=stat_url,
                      crawled=False,
                      )
                  gen_content_list.append(gen_content)
                  self.stdout.write(self.style.SUCCESS(e.name+' status: '+stat_url.encode('ascii','replace')))
              except:
                  pass
          if gen_content_list:
              GeneratedContent.objects.bulk_create(gen_content_list)
  </code></pre>
  
  <p>I get the following error:</p>
  
  <blockquote>
    <p>django.db.utils.IntegrityError: (1062, "Duplicate entry '19' for key
    'PRIMARY'")</p>
  </blockquote>
  
  <p>What am I doing wrong?</p>