title: Add array element if it's not contained already
body: <p>I am trying to add integer to an array if does not exist in the array, yet. But the following creates duplicate values:</p>
  
  <pre><code>User.update(
  {
      'topics': sequelize.fn('array_append', sequelize.col('topics'), topicId),
  },
  {where: {uuid: id}})
  </code></pre>
  
  <p>Is there an equivalent function in PostgreSQL/sequelize to MongoDB $addToSet?</p>