title: Mysql2::Error: Specified key was too long; max key length is 767 bytes for table having only 87 db fields
body: <p>I have rails model with table of 87 db columns but when I tried adding any extra db column, it failed</p>
  
  <pre><code>ArbitaryDatum.columns.count
   =&gt; 87
  
  ArbitaryDatum.table_name
   =&gt; "arbitary_data"
  
  ActiveRecord::Migration.add_column :arbitary_data, :attr51, :string
  
  -- add_column(:arbitary_data, :attr51, :string)
     (1.4ms)  ALTER TABLE `arbitary_data` ADD `attr51` varchar(255)
  Mysql2::Error: Specified key was too long; max key length is 767 bytes: ALTER TABLE `arbitary_data` ADD `attr51` varchar(255)
  ActiveRecord::StatementInvalid: Mysql2::Error: Specified key was too long; max key length is 767 bytes: ALTER TABLE `arbitary_data` ADD `attr51` varchar(255)
  </code></pre>
  
  <p>I checked which mysql version I am using,</p>
  
  <pre><code>mysql --version
  mysql  Ver 14.14 Distrib 5.5.62, for debian-linux-gnu (i686) using readline 6.3
  </code></pre>
  
  <p>I am not getting why it does not allow to add extra table column</p>
  
  <p><strong>Q.</strong> Why I am not getting <code>Mysql2::Error: Specified key was too long</code></p>