title: How to update a string set In DDB that is nested inside a map
body: <p>I am trying to add a string set onto another string set inside an item using the UpdateItem with the Javascript SDK</p>
  
  <p>My Parameters are such:</p>
  
  <pre><code>        var params = {
              Key: {
                  "MyKeyName" : {"S" : "MyKeyValue" }
              },
              TableName: "TableName",
              ExpressionAttributeNames: {
                  "#Name1" : "mapName"
              },
              ExpressionAttributeValues: {
                  ":Value1" : {"M" : {"StringSetName" : {
                                  "SS": ["ValueToAdd"]
                                  }
                              }
                  }
              },
              UpdateExpression: "ADD #Name1 :Value1"
          };
  </code></pre>
  
  <p>Now this doesn't work as ADD only works on Numbers and Sets and this set is nested under a map and it triggers this error:</p>
  
  <blockquote>
    <p>Incorrect operand type for operator or function; operator: ADD, operand type: MAP</p>
  </blockquote>
  
  <p>I tried changing the attribute name to mapName.M.StringSetName, and made the value <code>{"SS" : ["ValueToAdd"]</code>. That didn't trigger an error but it also didn't add the value to the set.</p>
  
  <p>Any thoughts on how to do this, sounds like it should be something close to what I am trying.</p>