title: rails migration partial index's where not working as expected
body: <p>i have a rails migration script:</p>
  <pre><code>add_index :site_tracking_codes, [:site_id, :is_published], where: &quot;is_published IS TRUE&quot;, unique: true
  </code></pre>
  <p>but it generates the index like so:</p>
  <pre><code>CREATE UNIQUE INDEX index_site_tracking_codes_on_site_id_and_is_published ON public.site_tracking_codes USING btree (site_id, is_published);
  </code></pre>
  <p>without the where condition.
  rails version: ~&gt; 4.2.11. postgreSQL 9.6 DB</p>