title: UNIQUE constraint failed on adding a new model field in django
body: <p>When i want to add a new field called <em>slug</em> to my <em>Post</em> model in <strong>Djnago</strong>, the migrate command will raise <code>UNIQUE constraint failed: new__chat_post.slug</code></p>
  
  <p>After that i remove that field from my model but the problem still exists. why?? and how to resolve this problem without deleting my whole table data??</p>
  
  <p>The database is <em>sqlite3</em> and the <em>Django</em> version is 2.2 .</p>
  
  <p>thanks.</p>
  
  <p>The model:</p>
  
  <pre><code>class Post(models.Model):    
      title = models.TextField(max_length=100)
      context = models.TextField(max_length=255)
      creation_date  = models.DateTimeField(default = timezone.now)
      author = models.ForeignKey(settings.AUTH_USER_MODEL,on_delete = models.CASCADE)
      slug = models.SlugField(default=["title"],unique=True)
  </code></pre>