title: Django, Postgres - column cannot be cast automatically to type integer
body: <p>In my database i have column:</p>
  
  <pre><code>currency = models.CharField(max_length=10, blank=True, null=True)
  </code></pre>
  
  <p>I want to change this column from <code>CharField</code> to <code>IntegerField</code>. So in <code>models.py</code> i change this:</p>
  
  <pre><code>currency = models.IntegerField(blank=True, null=True)
  </code></pre>
  
  <p>then i made migrations: <code>python manage.py makemigrations</code> and <code>python manage.py migrate</code>. After that actions it rise error:</p>
  
  <pre><code>django.db.utils.ProgrammingError: column "currency" cannot be cast automatically to type integer
  HINT:  Specify a USING expression to perform the conversion.
  </code></pre>
  
  <p>After that in pgAdmin3 console i made this changes:</p>
  
  <pre><code>ALTER TABLE my_table ALTER COLUMN currency TYPE integer USING (currency::integer);
  </code></pre>
  
  <p>But i still got that error, I tried to change all back, but error doesn't disappear. What i have to do to escape this error. Thank you</p>