title: ActiveAdmin get value from form text field
body: <p>I'm new in <code>Ruby</code> and make some simple admin using <code>ActiveAdmin</code>.
  I have a model <code>Question</code> which I want to create, fill and store to database, it has an attribute <code>themes</code> (array of <code>Theme</code> models). When user creates a new record he doesn't enter themes manually but provide some string and system will automatically parse it and find or create themes. So I have code like that:</p>
  
  <pre><code>form do |f|
      f.inputs "Questions Details" do
        f.input :question, as: :string
        f.input :autocomplete_themes, hint: "You should enter here multiple themes,
        divide them with `,` or `;`"
      end
      f.actions
  end
  </code></pre>
  
  <p>It creates a new field <code>autocomplete_themes</code> for entering string and it doesnt' exist in model <code>Question</code>. So what I want - is to get <code>autocomplete_themes</code> value like string and then use <code>split()</code> and my custom logic - but it gives an error.</p>
  
  <pre><code>before_create do |question|
      array = []
      puts "******"
      puts :autocomplete_themes.text
      themeTitles = :autocomplete_themes.split(",") #split(/,|;/)
      for title in themeTitles do
        theme = Theme.find_by(title: title)
        theme = Theme.create(title: title) unless theme
        array &lt;&lt; theme
      end
      question.themes = array
  end
  </code></pre>
  
  <p><b>Question:</b> how can I get <code>autocomplete_themes</code> value as string? Thx!</p>
  
  <p><b>Update:</b> as I understood <a href="https://stackoverflow.com/questions/24209661/activeadmin-custom-values-entered-into-form-fields">here</a> - it looks like the similiar case, but there was problem with setting default value to custom field, but I need to get its value from code.</p>