title: Add index with sort order?
body: <p>In Rails, how can I add an index in a migration to a Postgres database with a specific sort order?</p>
  
  <p>Ultimately wanting to pull off this query:</p>
  
  <pre><code>CREATE INDEX index_apps_kind_release_date_rating ON apps(kind, itunes_release_date DESC, rating_count DESC);
  </code></pre>
  
  <p>But right now in my migration I have this:</p>
  
  <pre><code>add_index :apps, [:kind, 'itunes_release_date desc', 'rating_count desc'], name: 'index_apps_kind_release_date_rating'
  </code></pre>
  
  <p>Which spits out this:</p>
  
  <pre><code>CREATE INDEX "index_apps_kind_release_date_rating" ON "apps" ("kind", "itunes_release_date desc", "rating_count desc")
  </code></pre>
  
  <p>Which errors out:</p>
  
  <pre><code>PG::Error: ERROR:  column "itunes_release_date desc" does not exist
  </code></pre>