How to update list of dictionary in dynamodb using boto3 with Conditional check

Question

I have dynamodb

• groupId is partition key
• empdetails is list
• dataDetails is the attribute name which i need to update
• if list is not there it has to create, but if its there then I need update the dictionary

I need to update the dataDetails list of dictionary for new dictionaries if and only if 'empId': 1001 not in the dataDetails

Code is below

new_obj = {'groupId':101, 'empId': 1001, 'empName': 'Joe'}

In my dataDetails list [{'empId': 1002, 'empName': 'Fra'}] is already there

So current dataDetails list is [ {'empId': 1002, 'empName': 'Fra'}, {'empId': 1001, 'empName': 'Joe'}]

dynamodb_resource = boto3.resource('dynamodb')
table = dynamodb_resource.Table('myTable')
groupId = new_object['groupId']
        data_details = {"empId": new_object['empId'],
                        "empName": new_object['empName']}
        result = table.update_item(
            Key={
                'groupId': groupId ,
            },
            UpdateExpression=
            'SET dataDetails = list_append(if_not_exists(dataDetails, :empty_list), :data_details)',
            ExpressionAttributeValues={
                ':data_details': [data_details], ':empty_list': []
            },
            ReturnValues="UPDATED_NEW"
        )

I got an error "errorMessage": "An error occurred (ValidationException) when calling the UpdateItem operation: Invalid UpdateExpression: An expression attribute value used in expression is not defined; attribute value: :empty_list",

After lot of trials I got solution [data_details], it should be in list. Updating in the above Code for reference

Is there any conditional check I can do like if 'empId': 1001 is already in the dataDetails so that it should not append to the dictionary?

Answer 1

There is more than the exception you are seeing wrong with your solution, but let's use 1 question on stackoverflow per problem as per the rules. The current exception is because you are using a variable that you have not declared, you can declare it like so:

dynamodb_resource = boto3.resource('dynamodb')
table = dynamodb_resource.Table('myTable')
groupId = new_object['groupId']
        data_details = {"empId": new_object['empId'],
                        "empName": new_object['empName']}
        result = table.update_item(
            Key={
                'groupId': groupId ,
            },
            UpdateExpression=
            'SET dataDetails = list_append(if_not_exists(dataDetails, :empty_list), :data_details)',
            ExpressionAttributeValues={
                ':data_details': [data_details], 
                ':empty_list': []
            },
            ReturnValues="UPDATED_NEW"
        )