Index on multiple columns in Ruby on Rails

I'm implementing functionality to track which articles a user has read.

  create_table "article", :force => true do |t|
    t.string   "title"
    t.text     "content"
  end

This is my migration so far:

create_table :user_views do |t|
  t.integer :user_id
  t.integer :article_id
end

The user_views table will always be queried to look for both columns, never only one. My question is how my index should look like. Is there a difference in the order of these tables, should there be some more options to it or whatever. My target DB is Postgres.

add_index(:user_views, [:article_id, :user_id])

Thanks.

UPDATE:
Because only one row containing the same values in both columns can exist (since in knowing if user_id HAS read article_id), should I consider the :unique option? If I'm not mistaken that means I don't have to do any checking on my own and simply make an insert every time a user visits an article.