title: Any string concatenation method in slick groupBy?
body: <p>I am trying to apply <code>.groupBy</code> in a Slick query</p>
  
  <pre><code>var q = (for {
        user &lt;- Users
        userSettings &lt;- UserSettings if user.id === userSettings.userId
      } yield (user, userSettings)).groupBy {
        case (users, userSettings) =&gt;
          (user.id, userSettings.controls)
      }.map {
        case (x, y) =&gt; (x._1, y.map(_._2.controls).???)
      }
  </code></pre>
  
  <p>If the <code>controls</code> column was an Integer or Long, I could apply <code>sum</code>, <code>avg</code> and other aggregate functions. But in this case <code>controls</code> is a string. How to group concatenate these strings so that records look like</p>
  
  <pre><code>-----------------------------------------
  |User ID    |User Controls              |
  -----------------------------------------
  |1          |left, right, up, down      |
  |2          |left, right                |
  -----------------------------------------
  </code></pre>
  
  <p>without applying <code>groupBy</code> records look like this</p>
  
  <pre><code>-----------------------------------------
  |User ID    |User Controls              |
  -----------------------------------------
  |1          |left                       |
  |1          |right                      |
  |1          |up                         |
  |1          |down                       |
  |2          |left                       |
  |2          |right                      |
  -----------------------------------------
  </code></pre>