title: Change column in Rails to be unique
body: <p>I know many similar questions have been asked before but I haven't found this question exactly (maybe it's just not possible).</p>
  
  <p>So I have a Column_A in my Rails table (using MySQL). Recently we've had the need to enforce uniqueness on this column.</p>
  
  <p>Is it possible to do a change on this column to make it unique? </p>
  
  <p>The only other solution I came up with is to create a temporary unique column and shuffle everything around. Which would be a pain.</p>
  
  <p>Thanks!</p>