title: Sunspot / Solr / Rails: Model Associations are not updating in the Index
body: <p>I have a Fieldnote model in my app, which has_many :activities attached to it through a table called :fieldnote_activities. I then define a searchable index this way:</p>
  
  <pre><code>searchable :auto_index =&gt; true, :auto_remove =&gt; true do
    integer :id
    integer :user_id, :references =&gt; User
  
    integer :activity_ids, :multiple =&gt; true do
      activities.map(&amp;:id)
    end
  
    text :observations
   end
  </code></pre>
  
  <p>And then I have a Search model to store / update searches. The search model thus also has its own associations with activities. I then perform my searches like this:</p>
  
  <pre><code>@search = Search.find(params[:id])
  @query  = Fieldnote.search do |query|
    query.keywords  @search.terms
  
    if @search.activities.map(&amp;:id).empty? == false
      query.with    :activity_ids, @search.activities.map(&amp;:id)
    end
  
  end
  @fieldnotes = @query.results
  </code></pre>
  
  <p>Now this all works GREAT. The problem is that if I change which activities that are associated with a fieldnote, the search results do not change because it appears the indices for that fieldnote do not change. I was under the impression that the :auto_index => true and :auto_remove => true flags when I define the searchable index would keep track of new associations (or deleted associations), but this appears not to be the case. How do I fix this?</p>