Document client abstraction in DynamoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this DynamoDB DocumentClient get operation?

Consider a DynamoDB table named Users with a primary key userId. The table contains an item with userId = '123' and name = 'Alice'. What will be the output of the following DocumentClient get call?

DynamoDB

const AWS = require('aws-sdk');
const docClient = new AWS.DynamoDB.DocumentClient();

const params = {
  TableName: 'Users',
  Key: { userId: '123' }
};

docClient.get(params).promise().then(data => console.log(data));

A {"Item": {"userId": "123", "name": "Alice"}}

B {"Items": [{"userId": "123", "name": "Alice"}]}

C {"userId": "123", "name": "Alice"}

D {"Count": 1, "Items": [{"userId": "123", "name": "Alice"}]}

💡 Hint

The get operation returns a single item inside the 'Item' property.

The DocumentClient get method returns an object with an 'Item' property containing the retrieved item. It does not return 'Items' or 'Count' for a single get.

📝 Syntax

intermediate

Which option correctly updates an attribute using DocumentClient update method?

You want to update the attribute age to 30 for the item with userId = '123' in the Users table. Which of the following params objects is syntactically correct for DocumentClient update?

A { TableName: 'Users', Key: { userId: '123' }, UpdateExpression: 'set age = :a', ExpressionAttributeValues: { ':a': '30' }, ReturnValues: 'UPDATED_NEW' }

B { TableName: 'Users', Key: { userId: '123' }, UpdateExpression: 'set age = a', ExpressionAttributeValues: { ':a': 30 }, ReturnValues: 'UPDATED_NEW' }

C { TableName: 'Users', Key: { userId: '123' }, UpdateExpression: 'set age = :a', ExpressionAttributeValues: { 'a': 30 }, ReturnValues: 'UPDATED_NEW' }

D { TableName: 'Users', Key: { userId: '123' }, UpdateExpression: 'set age = :a', ExpressionAttributeValues: { ':a': 30 }, ReturnValues: 'UPDATED_NEW' }

💡 Hint

ExpressionAttributeValues keys must start with a colon and match placeholders in UpdateExpression.

Option D correctly uses ':a' as a placeholder in both UpdateExpression and ExpressionAttributeValues with a numeric value 30. Option D misses the colon in UpdateExpression. Option D misses the colon in ExpressionAttributeValues key. Option D uses a string '30' instead of number 30 which may be valid but is not the intended numeric update.

❓ optimization

advanced

How to efficiently retrieve multiple items by keys using DocumentClient?

You need to get multiple items from the Users table by their userId keys: '123', '456', and '789'. Which approach is the most efficient and correct using DocumentClient?

A Use query with KeyConditionExpression userId IN ('123', '456', '789')

B Use batchGet with RequestItems: { Users: { Keys: [ { userId: '123' }, { userId: '456' }, { userId: '789' } ] } }

C Use scan with FilterExpression to filter userId IN ('123', '456', '789')

D Call get three times sequentially with each userId key

💡 Hint

BatchGet allows retrieving multiple items by keys in one request.

BatchGet is designed to retrieve multiple items by primary keys efficiently. Calling get multiple times is less efficient. Scan is expensive and not recommended for key lookups. Query does not support IN operator on partition key.

🔧 Debug

advanced

Why does this DocumentClient put operation fail with a ValidationException?

You try to put an item into the Users table with this code but get a ValidationException error. What is the cause?

DynamoDB

const params = {
  TableName: 'Users',
  Item: {
    userId: 123,
    name: 'Bob'
  }
};

await docClient.put(params).promise();

A The userId should be a string, not a number, because the table's primary key is a string.

B The TableName is case sensitive and should be 'users' instead of 'Users'.

C The Item property should be named Attributes instead of Item.

D The put method requires a ConditionExpression to succeed.

💡 Hint

Check the data type of the primary key value against the table schema.

DynamoDB primary keys are strongly typed. If the table's primary key is a string, providing a number causes ValidationException. Item is correct property name. TableName is case sensitive but usually 'Users' is correct if created so. ConditionExpression is optional.

🧠 Conceptual

expert

What is a key benefit of using DynamoDB DocumentClient abstraction?

Why do developers prefer using the DynamoDB DocumentClient abstraction over the low-level DynamoDB client?

A It encrypts data automatically before sending to DynamoDB.

B It provides built-in support for transactions without additional configuration.

C It automatically converts JavaScript objects to DynamoDB attribute value format and back, simplifying code.

D It allows querying tables without specifying keys or indexes.

💡 Hint

Think about how DocumentClient handles data types compared to the low-level client.

DocumentClient abstracts away the need to manually convert data to DynamoDB's attribute value format, making code simpler and easier to read. Transactions require explicit calls. Queries still require keys or indexes. Encryption is handled separately.