title: How to add `unique` constraint to already existing index by migration
body: <p>How can I add <code>unique: true</code> constraint to already existing index in Rails database?</p>
  
  <p>I tried to migrate by</p>
  
  <pre><code>  def change
      add_index :editabilities, [:user_id, :list_id], unique: true
    end
  </code></pre>
  
  <p>but migration fails with a error like this.</p>
  
  <blockquote>
    <p>Index name 'index_editabilities_on_user_id_and_list_id' on table
    'editabilities' already exists</p>
  </blockquote>
  
  <p>I'm using rails4 and postgresql.</p>