title: Adding a new Unique Field in an existing database table with existing values- Django1.7/MySql
body: <p>I have an existing database table.
  I want to add a new (Char)field to it. This field will have unique values.</p>
  
  <p>When I try to do so:</p>
  
  <pre><code>id = models.CharField(max_length=100, Unique=True)
  </code></pre>
  
  <p>I get integrity error.</p>
  
  <p>Some of the other things I have tried :</p>
  
  <pre><code>id = models.CharField(max_length=100, Unique=True,
  default="".join([random.random(), random.random()])))
  </code></pre>
  
  <p>and</p>
  
  <pre><code>id = models.CharField(max_length=100,
  default="".join([random.random(), random.random()])))
  </code></pre>
  
  <p>Same error.</p>
  
  <p>Is there a way around this? </p>