title: Validation for unique in form
body: <p>I have model and form like this</p>
  <pre><code>class Article(models.Model):
      key = models.CharField(max_length=20,null=False,unique=True)
  
  class ArticleForm(forms.ModelForm):
      class Meta:
         model = Article
      key = forms.CharField(required=True)
  </code></pre>
  <p>This <code>key</code> is unique in table, so I want to validate in form too.</p>
  <p>If I put the existing key, it comes to the</p>
  <p><code>1062, &quot;Duplicate entry 'inquiry' for key</code> error in yellow screen</p>
  <p>However it also doesn't work,</p>
  <pre><code>forms.CharField(required=True,unique=True)
  </code></pre>
  <p>How can I make this validation?</p>