title: Rails Migration changing column to use Postgres arrays
body: <p>I am trying to change a column in my database so that it can use the Postgres array data type.
  Currently the table column is of type string.</p>
  
  <p>I am using the following migration to convert it:</p>
  
  <pre><code>def change
    change_column :table, :dummy_column, :text, array: true, default: []
  end
  </code></pre>
  
  <p>But I get the following error:</p>
  
  <pre><code>bundle exec rake db:migrate
  rake aborted!
  An error has occurred, this and all later migrations canceled:
  
  PG::Error: ERROR:  column "dummy_column" cannot be cast automatically to type     character varying[]
  HINT:  Specify a USING expression to perform the conversion.
  : ALTER TABLE "table" ALTER COLUMN "dummy_column" TYPE character varying(255) 
  Tasks: TOP =&gt; db:migrate
  </code></pre>