title: how to auto populate form_form tag fields in rails in edit page
body: <p>i understand that if someone goes to an objects edit path, the form_for fields should populate. most of mines do except this one field, which has a has_many :through relationship with another table. how do you get it to pre populate with whatever the using typed in at creation?</p>
  
  <p>for example...</p>
  
  <pre><code>        &lt;div class="lesson_content"&gt;
              &lt;%= f.label :content %&gt;
              &lt;%= f.text_area :content %&gt;
          &lt;/div&gt;
  
          &lt;div class="tags"&gt;
              &lt;%= f.label :tag_names, "Tags" %&gt;
              &lt;%= f.text_field :tag_names, data: { autocomplete_source: tags_path} %&gt;
          &lt;/div&gt;
  </code></pre>
  
  <p>my lesson content will populate correctly, but not my tags field. i have a lessons table, tags table, and an intervening tags_relationship table.</p>
  
  <p>my lessons class is...</p>
  
  <pre><code>class Lesson &lt; ActiveRecord::Base
    attr_accessible :title, :desc, :content, :tag_names
    belongs_to :user
  
    has_many :tag_relationships, :autosave =&gt; true 
    has_many :tags, :through =&gt; :tag_relationships, :autosave =&gt; true
  </code></pre>
  
  <p>how do i get the tags field to populate? thanks</p>