title: Rails: Form_for fields without corresponding database columns
body: <p>I'm trying to do up a rails form_for that includes a checkbox for <code>:terms_and_conditions</code> (with <code>validates_presence_of</code>). <code>terms_and_conditions</code> has not been declared as a column in the corresponding database table.</p>
  
  <p>At the moment, when I submit the form and try to create a new model with <code>MyModel.new(params[:mymodel])</code>, I get an *'unknown attribute: terms_and_conditions'* error.</p>
  
  <p>So, just wondering: how do I run create a field for a column - and run validations on it - without inserting that field into the database?</p>
  
  <p>Many thanks...</p>