title: Adding a column as a foreign key gives ERROR column referenced in foreign key constraint does not exist
body: <p>I have the following set up,</p>
  
  <pre><code>CREATE TABLE auth_user ( id int PRIMARY KEY );
  CREATE TABLE links_chatpicmessage ();
  </code></pre>
  
  <p>I'm trying to <strong>add a column</strong> named <code>sender</code> to <code>links_chatpicmessage</code> which is a foreign key to another table called <code>auth_user</code>'s <code>id</code> column.</p>
  
  <p>To achieve the above, I'm trying the following on terminal:</p>
  
  <pre><code>ALTER TABLE links_chatpicmessage
    ADD FOREIGN KEY (sender)
    REFERENCES auth_user;
  </code></pre>
  
  <p>But this gives me an error: </p>
  
  <blockquote>
    <p>ERROR:  column "sender" referenced in foreign key constraint does not
    exist</p>
  </blockquote>
  
  <p>How do I fix this?</p>