title: Can't add foreign key after a index drop
body: <p>So real quick what I did:</p>
  
  <ol>
  <li>Realisted didn't need unique key but still foreign key</li>
  <li><p>First needed to drop the foreign key </p>
  
  <p><code>ALTER TABLE groups DROP FOREIGN KEY group_information_id</code></p></li>
  <li><p>Then dropped the unique key with </p>
  
  <p><code>ALTER TABLE groups DROP INDEX group_information_id</code></p></li>
  <li><p>Now I want to add the foreign key again with </p>
  
  <p><code>ALTER TABLE groups ADD CONSTRAINT group_information_id FOREIGN KEY (group_id) REFERENCES group_information.group_id</code></p></li>
  <li><p>Get the error (this is tanslated sorry if i'm not accurate, tell me if you don't understand): </p></li>
  </ol>
  
  <blockquote>
    <p>Can not add foreign key</p>
  </blockquote>
  
  <p>Why is this happening, and how can I fix this so I can add a <code>foreign key</code>?</p>
  
  <p>Engine <code>innodb</code> used</p>