title: Rails 4 - Mysql2::Error: Index column size too large
body: <p>In rails 4.2.4, I am getting an error like <code>Mysql2::Error: Index column size too large. The maximum column size is 767 bytes.</code> while running the migration command.</p>
  
  <p>Mysql gem is,</p>
  
  <pre><code>mysql2 (0.3.20, 0.3.18)
  </code></pre>
  
  <p>When I run this <code>ActiveRecord::Base.connection.collation</code> in console, then I got the output like <code>"utf8_general_ci"</code></p>
  
  <p>How can I fix this issue? Please help me.</p>