DynamoDB update_item覆盖项目

Question

Update Item is overwriting my item instead of adding a new one to the list. Following the directions here , I should get a updated list.

My Code:

dynamodb = boto3.resource('dynamodb', region_name='us-west-2', endpoint_url="http://localhost:8000")

table = dynamodb.Table('WishListTest')

device = input('What is the Item being requested?\n')
device = device.upper()

aliasInput = input('What is the Alias of the user?\n')
aliasInput = aliasInput.upper()

date = int((time.strftime("%d%m%Y")))
response = table.update_item(
    Key={
        'Device': device,
    },
    UpdateExpression="set RequestList.Alias = :r, RequestList.Createddate = :d, AvailableQuanity = :a, "
                 "ReserveQuanity = :q",
    ExpressionAttributeValues={
        ':r': aliasInput,
        ':d': date,
        ':a': 0,
        ':q': 0
    }
)
print("PutItem succeeded:")
print(json.dumps(response, indent=4, cls=DecimalEncoder))

My output looks like this:

{"ReserveQuanity": 0, "AvailableQuanity": 0, "Device": "DELL", "RequestList": {"CreatedDate": 19122017, "Alias": "JONJP", "Createddate": 27122017}}

It overwrites it so long as I maintain the same Device name. Which it is supposed to just add another Alias , Createddate , ReserveQuanity , and AvailableQuanity to the end.

Answer 1

First of all you need to ensure that all the attributes you want to add are either lists or sets .

If you use a list you need to use the list_append function in the update expression, as stated in the boto3 documentation . Otherwise you'd just overwrite the pre-existing value as you do right now:

SET - Adds one or more attributes and values to an item. If any of these attribute already exist, they are replaced by the new values. You can also use SET to add or subtract from an attribute that is of type Number. For example: SET myNum = myNum + :val SET supports the following functions:

• if_not_exists (path, operand) - if the item does not contain an attribute at the specified path, then if_not_exists evaluates to operand; otherwise, it evaluates to path. You can use this function to avoid overwriting an attribute that may already be present in the item.
• list_append (operand, operand) - evaluates to a list with a new element added to it. You can append the new element to the start or the end of the list by reversing the order of the operands.

If you use a set you don't need the list_append function, but have to use the ADD action instead of the SET action:

ADD - Adds the specified value to the item, if the attribute does not already exist. If the attribute does exist, then the behavior of ADD depends on the data type of the attribute:

• If the existing data type is a set and if Value is also a set, then Value is added to the existing set.
• If the existing data type is a set and if Value is also a set, then Value is added to the existing set. For example, if the attribute value is the set [1,2] , and the ADD action specified [3] , then the final attribute value is [1,2,3] . An error occurs if an ADD action is specified for a set attribute and the attribute type specified does not match the existing set type. Both sets must have the same primitive data type. For example, if the existing data type is a set of strings, the Value must also be a set of strings.