title: Migration with a decimal number with 2 trailing digits
body: <pre><code>Rails 3.2
  MySQL gem
  </code></pre>
  
  <p>I have the following in my migration:</p>
  
  <pre><code>t.decimal :pre_tax_total, default: nil, scale: 2
  t.decimal :post_tax_total, default: nil, scale: 2
  </code></pre>
  
  <p>Based on what I read, scale:2 will produce a decimal with 2 trailing digits.</p>
  
  <p>When I run the migration, and look at the table structure, I see the following:</p>
  
  <pre><code>pre_tax_total   decimal(10,0) 
  post_tax_total  decimal(10,0)
  </code></pre>
  
  <p>Which means the values are getting truncated by the MySQL server. What is the ActiveRecord syntax to create these columns as decimal(10,2)?   </p>