title: Formtastic/ActiveAdmin set a default value for a select input
body: <p>I am working on a custom form in ActiveAdmin which I decided to use it for adding/editing to follow the <code>DRY</code> principle so I need to populate it if the user uses it for editing a record ( which isn't a DB record ).</p>
  
  <p>So the problem is I have those inputs :</p>
  
  <pre><code>f.input :model_id, as: :select, collection: Model.all.map { |m| [m.id.to_s + ' - ' + m.name, m.id] }, input_html: { required: true }
  f.input :enabled, as: :select, collection: {'Yes': true, 'No': false}, input_html: { required: true }
  </code></pre>
  
  <p>And I'd like to set a default value for them if I'm using the form for editing but I failed to know how because everybody is talking about using <code>belongs_to</code> or a DB relation and <code>ActiveAdmin</code> will take care of the default value for you which isn't applicable for my case because it's not a DB record and I don't have an <code>ActiveRecord Model</code> for it.</p>
  
  <p>Even the <a href="http://www.rubydoc.info/github/justinfrench/formtastic/Formtastic/Inputs/SelectInput" rel="noreferrer">official docs</a> of <code>Formtastic</code> didn't help.</p>