title: Dynamodb UpdateExpression adding new map object to a list of existing Item
body: <p>I currently have a <strong>List</strong> attribute in my dynamodb table:</p>
  
  <pre><code> name: "Test User"
   recommendations: []
  </code></pre>
  
  <p>I would like to <strong>add</strong> new item to the recommendations attribute using the UpdateExpression</p>
  
  <pre><code>const params = {
      TableName: 'insiders',
      Key:{
        "uuid": event.uuid, // WHERE uuid is event.uuid
      },
      UpdateExpression: "SET #recommendations = :recommendation",
      ExpressionAttributeNames: {
        "#recommendations": "recommendations",
      },
      ExpressionAttributeValues: {
        ":recommendation": [{ 
          "uuid":  `ir_${uuidv4()}`, 
          "recommendation": event.recommendation
        }]
      },
      ReturnValues:"UPDATE_NEW"
    };
  
    dynamodb.update(params, function(err, data) { }
  </code></pre>
  
  <p>I managed to add an object map to recommendations list but when I want to add another one it will replace the object in the recommendation list.</p>
  
  <p>I also tried to use the <strong>ADD</strong> in UpdateExpression</p>
  
  <pre><code>const params = {
      TableName: 'insiders',
      Key:{
        "uuid": event.uuid,
      },
      UpdateExpression: "ADD #recommendations :recommendation",
      ExpressionAttributeNames: {
        "#recommendations": "recommendations",
      },
      ExpressionAttributeValues: {
        ":recommendation": [{ 
          "uuid":  `ir_${uuidv4()}`, 
          "recommendation": event.recommendation, 
        }]
      },
      ReturnValues:"ALL_NEW"
    };
  
     dynamodb.update(params, function(err, data) { }
  </code></pre>
  
  <p>but Im getting an error </p>
  
  <pre><code>"Invalid UpdateExpression: Incorrect operand type for operator or function; operator: ADD, operand type: LIST",
  </code></pre>