Implementation

• Updating an item

  When updating an item in your application code, focus on four elements:
  

  • The Key that identifies the item you want to update.
  • Expression attribute names, which are placeholders for the attributes you want to update.
  • Expression attribute values, which are placeholders for the values to which you want to update your attributes.
  • The update expression, which uses the expression attribute names and expression attribute values to declare the updates you want. Imagine that you have recently added an audiobook format for John Grisham's The Rainmaker, and you want to update our DynamoDB table to reflect that change. The UpdateItem API call would look as follows.
    

  # The UpdateItem API allows you to update a particular item as identified by its key.
  resp = table.update_item(
      Key={"Author": "John Grisham", "Title": "The Rainmaker"},
      # Expression attribute names specify placeholders for attribute names to use in your update expressions.
      ExpressionAttributeNames={
          "#formats": "Formats",
          "#audiobook": "Audiobook",
      },
      # Expression attribute values specify placeholders for attribute values to use in your update expressions.
      ExpressionAttributeValues={
          ":id": "8WE3KPTP",
      },
      # UpdateExpression declares the updates you want to perform on your item.
      # For more details about update expressions, see https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.UpdateExpressions.html
      UpdateExpression="SET #formats.#audiobook = :id",
  )

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  Your UpdateItem API call has each of the four elements. First, it specifies the Key of the item it wants to operate on by providing the Author as John Grisham and the Title as The Rainmaker.

  Second, you specify the expression attribute names that you will use in your update expression. You are modifying the Formats attribute, which is a map, so you need to define expression attribute names for both the attribute (Formats) and the key within your map (Audiobook).

  Finally, you write the update expression that you want to apply in your UpdateItem API call. This update expression uses both the expression attribute names and expression attribute values as previously defined. Your expression of SET #formats.#audiobook = :id tells DynamoDB to set the Audiobook key of the Formats attribute to the given ID value of 8WE3KPTP. 

  Third, you state the value you want to use in the expression attribute values. In your Formats map, the value for a format is the item number in your inventory system. In this example, the value is 8WE3KPTP.

  The update_item.py script makes the UpdateItem API call shown in the code block earlier in the module. To demonstrate the difference made by the call, the script first retrieves and prints the item before the update call is made. It then makes the UpdateItem API call. Finally, the script retrieves and prints the item a second time to show the difference.

  You need to add a few lines of code before you run the script. Open update_item.py in Cloud9, and paste the following three lines at the top of the script:

  

  Close

  

  import boto3
  dynamodb = boto3.resource('dynamodb', region_name='us-east-1')
  table = dynamodb.Table('Books')

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  Next, run the following command:
  

  $ python update_item.py
  

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  Now, run the following command:

  $ python get_item.py

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  You should see the following output:

  {'Title': 'The Rainmaker', 'Formats': {'Audiobook': '8WE3KPTP', 'Hardcover': 'J4SUKVGU', 'Paperback': 'D7YF4FCX'}, 'Author': 'John Grisham', 'Category': 'Suspense'}

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  As you can see, The Rainmaker has a new Audiobook format after the update has been applied.
  
  In this module, you learned how to update an existing item with the UpdateItem API. In the next module, you learn how to clean up the resources you created to avoid additional charges.