title: ActiveRecord column cannot be cast automatically to type numeric
body: <p>I need to change the data type of a column in a DB from <code>:string</code> to <code>:decimal</code>. All values are in the following format: <code>"129.99"</code>, so converting them should not raise any errors.</p>
  
  <p>To do so I wrote the following migration:</p>
  
  <pre><code>def change
    change_column :my_table, :target_column, :decimal
  end
  </code></pre>
  
  <p>When I execute this it shows me the following error together with a hint on how to fix it:</p>
  
  <pre><code>PG::DatatypeMismatch: ERROR:  column "target_column" cannot be cast automatically to type numeric
  HINT:  You might need to specify "USING target_column::numeric".
  </code></pre>
  
  <p>However, I can't seem to find any documentation about how to do this, so the hint doesn't really help me.</p>
  
  <p>Whats the best way to perform this migration?</p>