title: Rails Simpleform with non-model inputs
body: <p>I have a normal form using simpleform. Now I'd like to add an input that does not have any corresponding field in the model, it will be processed by the controller. I tried</p>
  
  <pre><code>&lt;%= simple_form_for @obj do |f| %&gt;
    &lt;%= f.input :name %&gt;
    &lt;%= f.input :attr, as: :string %&gt;   &lt;-- should just send "attr" as post data
  &lt;% end %&gt;
  </code></pre>
  
  <p>but this gives a <code>Method not found: attr_not_in_obj</code> error. I could obviously use the standard rails helpers, but then I will miss all of the simpleform HTML around the input, and copying doesn't quite seem right.</p>
  
  <p>In short:
  I'm looking for something like simpleform version of rails tag helpers, without any connection to a model. How do I add inputs that do not correspond to model attributes? </p>