Conditional expressions in DynamoDB - Mini Project: Build & Apply

Using Conditional Expressions in DynamoDB

📖 Scenario: You are managing a simple inventory system for a small store. You want to update the stock quantity of items only if the current stock is above a certain threshold to avoid negative stock.

🎯 Goal: Build a DynamoDB update operation using a conditional expression that updates the stock quantity only if the current stock is greater than 5.

📋 What You'll Learn

Create a DynamoDB table named Inventory with a primary key ItemId.

Insert an item with ItemId 'A101' and Stock 10.

Define a variable min_stock with value 5.

Write an update operation with a conditional expression that updates Stock only if current Stock is greater than min_stock.

💡 Why This Matters

🌍 Real World

Conditional expressions help prevent unwanted data changes in databases, such as avoiding negative stock in inventory systems.

💼 Career

Understanding conditional expressions is important for database administrators and backend developers to maintain data integrity and implement business rules.

1

Create the DynamoDB table and insert an item

Create a DynamoDB table named Inventory with primary key ItemId. Insert an item with ItemId set to 'A101' and Stock set to 10.

DynamoDB

# Create table Inventory with primary key ItemId
# Insert item with ItemId 'A101' and Stock 10

Need a hint?

Define Inventory as a dictionary with table details. Then create items dictionary with ItemId and Stock.

2

Define the minimum stock threshold

Define a variable called min_stock and set it to 5.

DynamoDB

Inventory = {
    'TableName': 'Inventory',
    'KeySchema': [
        {'AttributeName': 'ItemId', 'KeyType': 'HASH'}
    ],
    'AttributeDefinitions': [
        {'AttributeName': 'ItemId', 'AttributeType': 'S'}
    ],
    'ProvisionedThroughput': {
        'ReadCapacityUnits': 5,
        'WriteCapacityUnits': 5
    }
}

items = {
    'ItemId': 'A101',
    'Stock': 10
}

# Define min_stock variable
# Your code here

Need a hint?

Just create a variable named min_stock and assign it the value 5.

3

Write the update operation with conditional expression

Write a dictionary called update_params that represents an update operation on the Inventory table. It should update the Stock attribute to 8 for the item with ItemId 'A101' only if the current Stock is greater than min_stock. Use the key ConditionExpression with the value Stock > :min_stock and define ExpressionAttributeValues accordingly.

DynamoDB

Inventory = {
    'TableName': 'Inventory',
    'KeySchema': [
        {'AttributeName': 'ItemId', 'KeyType': 'HASH'}
    ],
    'AttributeDefinitions': [
        {'AttributeName': 'ItemId', 'AttributeType': 'S'}
    ],
    'ProvisionedThroughput': {
        'ReadCapacityUnits': 5,
        'WriteCapacityUnits': 5
    }
}

items = {
    'ItemId': 'A101',
    'Stock': 10
}

min_stock = 5

# Create update_params dictionary with ConditionExpression and ExpressionAttributeValues
# Your code here

Need a hint?

Use UpdateExpression to set Stock to 8. Use ConditionExpression to check Stock > :min_stock. Define ExpressionAttributeValues with :new_stock and :min_stock.

4

Complete the update operation with return values

Add the key ReturnValues to update_params and set it to ALL_NEW to return the updated item after the update.

DynamoDB

Inventory = {
    'TableName': 'Inventory',
    'KeySchema': [
        {'AttributeName': 'ItemId', 'KeyType': 'HASH'}
    ],
    'AttributeDefinitions': [
        {'AttributeName': 'ItemId', 'AttributeType': 'S'}
    ],
    'ProvisionedThroughput': {
        'ReadCapacityUnits': 5,
        'WriteCapacityUnits': 5
    }
}

items = {
    'ItemId': 'A101',
    'Stock': 10
}

min_stock = 5

update_params = {
    'TableName': 'Inventory',
    'Key': {'ItemId': 'A101'},
    'UpdateExpression': 'SET Stock = :new_stock',
    'ConditionExpression': 'Stock > :min_stock',
    'ExpressionAttributeValues': {
        ':new_stock': 8,
        ':min_stock': min_stock
    }
}

# Add ReturnValues key to update_params
# Your code here

Need a hint?

Add 'ReturnValues': 'ALL_NEW' inside the update_params dictionary.