title: Cant update Dynamo Db table , getting ValidationException
body: <p>I need to update my dynamo db table by using only partition key. But i got validation exeption. 
  I have created a table with 3 fields. </p>
  
  <ol>
  <li>id (Partition Key)</li>
  <li>name (Sort Key)</li>
  <li>age</li>
  </ol>
  
  <p>Then i have triyed to update age field using only id.(tryied to modify age 30 to 40) this is my code</p>
  
  <pre><code>var AWS = require("aws-sdk");
  
  AWS.config.update({
      region: "us-east-1",
  
  });
  
  var params = {
      TableName: 'test',
      Key: { id: '100' },
      UpdateExpression: 'set #age = :age ',
      ConditionExpression: '#age = :testAge',
      ExpressionAttributeNames: { '#age': 'age' },
      ExpressionAttributeValues: { ':age': '40', ':testAge': '30' }
  };
  
  var docClient = new AWS.DynamoDB.DocumentClient();
  docClient.update(params, function (err, data) {
      if (err) {
          console.log(err);
      }
      else {
          console.log(data);
      }
  });
  </code></pre>
  
  <p>But i got error like this. </p>
  
  <pre><code>{ [ValidationException: The provided key element does not match the schema]
    message: 'The provided key element does not match the schema',
    code: 'ValidationException',
    time: Thu Nov 17 2016 22:38:01 GMT+0530 (IST),
    requestId: '34PNMFM6CEACQIRHTSV77OI0JRVV4KQNSO5AEMVJF66Q9ASUAAJG',
    statusCode: 400,
    retryable: false,
    retryDelay: 0 }
  </code></pre>
  
  <p>After getting error, i modified my params variable like this</p>
  
  <pre><code> var params = {
          TableName: 'test',
          Key: { id: '100',name: 'manaf' },
          UpdateExpression: 'set #age = :age ',
          ConditionExpression: '#age = :testAge',
          ExpressionAttributeNames: { '#age': 'age' },
          ExpressionAttributeValues: { ':age': '40', ':testAge': '30' }
      };
  </code></pre>
  
  <p>Using this, updation is successfully completed. How to update table using without sort key?</p>