title: Creating a unique constraint on two columns together in Ecto
body: <p>How do you create a unique index on two columns in Ecto, which would correspond to this:</p>
  
  <pre><code>CREATE TABLE someTable (
      col1 int NOT NULL,
      col2 int NOT NULL,
      primary key (col1, col2)
  )
  </code></pre>
  
  <p>?</p>