title: Auto-increment field of six digits
body: <p>I have a table with an "identifier" attribute. 
  I need this attribute to be unique and auto-incremented by one (length of the attribute must be six digits).</p>
  
  <p>For example the first time I persist an entity, the identifier should be 000001 and the second one 000002 and so on.</p>
  
  <p>Could you please tell me how to implement this requirement?</p>
  
  <p>Thanks in advance.</p>