title: Build association while initializing to satisfy delegates
body: <p>I have this minimal example model: </p>
  
  <pre><code>class Book &lt; ActiveRecord::Base
    belongs_to :author
    delegate :name, prefix: true, to: author
  
    after_initialize { author ||= Author.new } 
  end
  </code></pre>
  
  <p>After posting form data my framework (yes, ActiveAdmin) does this: <code>Book.new {author_name: 'Some Dude'}</code> which results in <code>author_name</code> not being written since the <code>after_initialize</code> callback is only called after initialization of <code>Book</code>. </p>
  
  <p>How can I build the association "before" or "while" initializing? Any good pattern? </p>