Updating Sub-Item in Item in DynamoDB using Python

Question

Hi want to add continuously to an sub-item which is initially declared empty and later will be updated.

Here is the code -

dynamodb = boto3.resource('dynamodb',endpoint_url='http://localhost:8000')
# Creating the Table
table = dynamodb.create_table(
            TableName='TableN',
            KeySchema=[
                           {
                           'AttributeName': 'name',
                           'KeyType': 'HASH'
                           }
                       ],
            AttributeDefinitions=[

                            {
                                'AttributeName': 'name',
                                'AttributeType': 'S'
                            }
                        ],
            ProvisionedThroughput={
                        'ReadCapacityUnits': 1000,
                        'WriteCapacityUnits': 1000
                        }
        )
# Updating with the Initial Values
table = dynamodb.Table('TableN')
IntValues = {
              "datapoints": [], 
               "attributes": {"host": "server1", "customer": "Ast1"}, 
                "name": "B001_P001"
             }

table.put_item(Item=IntValues)

Now i want to update the "datapoints" with the set of values -

datapoint1 = {"x1":1,"x2":"OK","x3":"23.123"}
datapoint2 = {"x1":3,"x2":"ON","x3":"56.123"}
datapoint3 = {"x1":5,"x2":"OFF","x3":"78.123"}

So that final value should look like -

{
    "name" : "B001_P001",
    "datapoints" : [
        {
          "x1" : 1,
          "x2" : "OK",
          "x3":"23.123"
        },
        {
          "x1" : 1,
          "x2" : "OK",
          "x3":"23.123"
        },
        {
          "x1" : 1,
          "x2" : "OK",
          "x3":"23.123"
        }]
        "attributes": {"host": "server1", "customer": "Ast1"}
}

How can i update the existing sub-item without removing or creating again the entire item ?

Chandra Sekhar K · Accepted Answer · 2018-09-17 09:51:38Z

1

I have used the update_item to update each datapoint-

After the inital update created as shown in question using the put_item, i have used the update_item to update the attributes.

I have used for loop to pass the individual datapoints -

ulist = datapoint1     
table.update_item(
                  Key={'name':'TableN'},
                  UpdateExpression= 'SET #datapoints = list_append(#datapoints,:val1)',
                  ExpressionAttributeNames={'#datapoints': 'datapoints'},
                  ExpressionAttributeValues={':val1':ulist}
                 )

answered Sep 17, 2018 at 9:51

Chandra Sekhar K

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