title: MySQL is not allowing ON UPDATE CURRENT_TIMESTAMP for a DATETIME field
body: <p>I have seen a lot of related questions, but I cannot place my finger on this specific question:</p>
  
  <p>I have a MySQL table with both a TIMESTAMP (for when the field was created) and a DATETIME (for each time the field gets updated).  It looks like this:</p>
  
  <pre><code>CREATE TABLE 'vis' (
  ID BIGINT PRIMARY KEY AUTO_INCREMENT NOT NULL,
  ENTRY VARCHAR(255),
  AUTHOR VARCHAR(255),
  CREATED_AT TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
  UPDATED_AT DATETIME ON UPDATE CURRENT_TIMESTAMP,
  UPDATED_BY VARCHAR(255)
  )
  </code></pre>
  
  <p>When I try this though, the error I am getting is: <code>(SQL Error: 1294 SQL State: HY000) - Invalid ON UPDATE clause for 'updated_at' field</code></p>
  
  <p>Everywhere I have read (even on Stack Overflow) suggests I should be able to do this, yet I am getting this error.  Perhaps there is another way to have a field that automatically updates the time each time I update it?</p>
  
  <p>I am using MySQL Server 5.5.</p>