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.

Practice

1. What is the main benefit of using the DynamoDB Document Client abstraction?

easy

A. It requires you to write complex JSON expressions for queries.

B. It allows you to work with plain JavaScript objects without manual data conversion.

C. It only supports string data types in DynamoDB.

D. It disables the use of PutCommand and GetCommand.

Solution

1. Step 1: Understand Document Client purpose

   The Document Client simplifies interaction by letting you use normal JavaScript objects.

2. Step 2: Compare options

   The incorrect options describe limitations or incorrect features, while the correct one states the main benefit.

3. Final Answer:

   It allows you to work with plain JavaScript objects without manual data conversion. -> Option B

4. Quick Check:

   Document Client simplifies data handling = A [OK]

Hint: Document Client hides data format details, use plain objects [OK]

Common Mistakes:

• Thinking you must manually convert data formats
• Believing it only supports strings
• Confusing commands it supports

2. Which of the following is the correct way to import the DynamoDB Document Client in JavaScript?

easy

A. import DynamoDBDocumentClient from 'aws-sdk';

B. const DynamoDBDocumentClient = require('aws-sdk/lib-dynamodb');

C. import { DynamoDBDocumentClient } from "@aws-sdk/lib-dynamodb";

D. const { DynamoDBDocumentClient } = require('@aws-sdk/client-dynamodb');

Solution

1. Step 1: Identify correct import syntax

   The Document Client is imported from "@aws-sdk/lib-dynamodb" using ES module syntax.

2. Step 2: Eliminate incorrect options

   The CommonJS require uses the wrong path; importing from 'aws-sdk' uses the wrong package; destructuring from '@aws-sdk/client-dynamodb' uses the wrong package.

3. Final Answer:

   import { DynamoDBDocumentClient } from "@aws-sdk/lib-dynamodb"; -> Option C

4. Quick Check:

   Correct ES module import from lib-dynamodb = A [OK]

Hint: Use ES module import from '@aws-sdk/lib-dynamodb' [OK]

Common Mistakes:

• Importing from '@aws-sdk/client-dynamodb' instead
• Using require with wrong package
• Missing curly braces in import

3. Given the following code snippet using DynamoDB Document Client, what will be the output if the item exists?

const params = { TableName: "Users", Key: { userId: "123" } };
const command = new GetCommand(params);
const data = await docClient.send(command);
console.log(data.Item);

medium

A. An object representing the user with userId '123', e.g., { userId: '123', name: 'Alice' }

B. A string containing the userId '123'

C. An error because GetCommand is not supported by Document Client

D. Undefined, because data.Item is not returned

Solution

1. Step 1: Understand GetCommand with Document Client

   GetCommand returns an object with an Item property containing the requested item if it exists.

2. Step 2: Analyze console.log output

   Logging data.Item prints the full object stored for userId '123', not just a string or undefined.

3. Final Answer:

   An object representing the user with userId '123', e.g., { userId: '123', name: 'Alice' } -> Option A

4. Quick Check:

   GetCommand returns item object = B [OK]

Hint: GetCommand returns full item object in data.Item [OK]

Common Mistakes:

• Expecting only the key value as output
• Thinking GetCommand is unsupported
• Assuming data.Item is undefined

4. Identify the error in this DynamoDB Document Client code snippet:

const params = { TableName: "Products", Item: { id: 101, name: "Pen" } };
const command = new GetCommand(params);
const result = await docClient.send(command);
console.log(result);

medium

A. Using GetCommand with an Item parameter instead of Key

B. Missing await keyword before docClient.send

C. TableName should be lowercase

D. Item should be a string, not an object

Solution

1. Step 1: Check command and parameters

   GetCommand requires a Key parameter to identify the item, not an Item parameter.

2. Step 2: Validate other options

   Await is present, TableName is case-sensitive but correct, and Item must be an object for PutCommand, not GetCommand.

3. Final Answer:

   Using GetCommand with an Item parameter instead of Key -> Option A

4. Quick Check:

   GetCommand needs Key, not Item = D [OK]

Hint: GetCommand uses Key, PutCommand uses Item [OK]

Common Mistakes:

• Confusing Item and Key parameters
• Ignoring case sensitivity of TableName
• Forgetting await keyword

5. You want to add a new user to the DynamoDB table 'Users' using the Document Client. Which code snippet correctly performs this operation?

hard

A. const params = { TableName: 'Users', Key: { userId: 'u123' }, Item: { name: 'Bob' } };
const command = new PutCommand(params);
await docClient.send(command);

B. const params = { TableName: 'Users', Key: { userId: 'u123' } };
const command = new PutCommand(params);
await docClient.send(command);

C. const params = { TableName: 'Users', Item: { userId: 'u123' } };
const command = new GetCommand(params);
await docClient.send(command);

D. const params = { TableName: 'Users', Item: { userId: 'u123', name: 'Bob' } };
const command = new PutCommand(params);
await docClient.send(command);

Solution

1. Step 1: Understand PutCommand parameters

   PutCommand requires an Item parameter with the full object to insert, including the key attributes.

2. Step 2: Analyze each option

   Using both Key and Item is incorrect; using only Key omits the Item; using GetCommand retrieves instead of inserts; the correct snippet uses Item containing both key and attributes.

3. Final Answer:

   const params = { TableName: 'Users', Item: { userId: 'u123', name: 'Bob' } }; const command = new PutCommand(params); await docClient.send(command); -> Option D

4. Quick Check:

   PutCommand with Item including key = C [OK]

Hint: PutCommand needs full Item object including keys [OK]

Common Mistakes:

• Using Key instead of Item for PutCommand
• Mixing GetCommand with PutCommand parameters
• Omitting key attributes in Item