title: Mapping `has_many` in active admin
body: <p>I am trying to map the <code>name</code> attribute from an array of ID's in my index view within active admin.</p>
  
  <p>I have</p>
  
  <p><strong>Sector Model</strong></p>
  
  <pre><code>has_many :portfolio_sectors
  has_many :portfolios, through: :portfolio_sectors
  </code></pre>
  
  <p><strong>Portfolio Model</strong></p>
  
  <pre><code>has_many :portfolio_sectors
  has_many :sectors, through: :portfolio_sectors
  </code></pre>
  
  <p><strong>Portfolio Sector</strong></p>
  
  <pre><code>belongs_to :portfolio
  belongs_to :sector
  </code></pre>
  
  <p>Within active admin, at the moment, I have</p>
  
  <pre><code># Index
  index do
    selectable_column
    column :sector_ids
    actions
  end
  </code></pre>
  
  <p>which will return</p>
  
  <pre><code>[2, 3]
  </code></pre>
  
  <p>I have the ID's of my sectors. How can I display the <code>name</code> attribute? Using <code>map</code> gives me</p>
  
  <pre><code>undefined method `map` for fixnum 2
  </code></pre>
  
  <p>How do I access <code>name</code>?</p>