title: DynamoDb: Perform updateExpression with string concatenation
body: <p>Is there a way to concatenate string via UpdateExpressions? Let me explain better, for example, if a record has an id, thread and message, and for some reason, I intend to update one attribute with an information that already exists in the record, I could perform an expression like:</p>
  
  <pre><code>updateExpression = `SET #thread2 = #thread
  </code></pre>
  
  <p>it works, but unfortunately I couldn't do something beyond this, like a concatenation:</p>
  
  <pre><code>updateExpression = `SET #department = #thread + #id
  </code></pre>
  
  <p>or</p>
  
  <pre><code>updateExpression = `SET #department = #thread.#id
  </code></pre>
  
  <p>or</p>
  
  <pre><code>updateExpression = `SET #department = #thread#id
  </code></pre>
  
  <p>Some ideas? If not, I guess it could be very useful to avoid a GET before an UPDATE operation just to get an existent data and concatenate with some other stuff.</p>