title: Displaying has_many through association as column in Active Admin index
body: <p>How do I display a list of has_many_through associations with the association as the column headings and the through: value as an entry in the table row? </p>
  
  <p>I have 3 models: </p>
  
  <pre><code>class Jobs 
    attr_accesor :title 
  
    has_many :scores 
    has_many :factors, through: :scores 
  end 
  
  class Scores 
    attr_accesor :score 
  
    belongs_to :job 
    belongs_to :factor 
  end 
  
  class Factor 
    attr_accesor :name 
    has_many :scores 
    has_many :jobs, through: :scores 
  end 
  </code></pre>
  
  <p>I want to be able to show, in the Jobs index, a row for each Job, the title of each Factor as a column heading, and the scores of each Job as the value in the cell.  </p>
  
  <p>I would expect to have to do something like this in the <code>app/admin/jobs.rb</code> file:</p>
  
  <pre><code>index do |jobs|
    column :title
    jobs.scores.each do |score|
      column(score.factor.name) { |score| score.score }
    end
  end
  </code></pre>
  
  <p>And get output like this: </p>
  
  <pre><code>Job              |  Education  |  Experience  |  Leadership  |  ...  |
  - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  CEO              |     600     |     720      |     580      |  ...  |
  Admin Assistant  |     210     |     200      |     150      |  ...  |
  </code></pre>
  
  <p>But activeadmin doesn't seem to like the <code>jobs.scores.each</code> line, giving me the following error:</p>
  
  <pre><code>undefined method `scores' for 
  #&lt;ActiveAdmin::Views::IndexAsTable::IndexTableFor:0x00000104d1dad0&gt;
  </code></pre>