How to Append value to an empty list attribute in AWS DynamodB

Question

I am trying to append a message to an empty list in AWS DynamoDB.

Here is the error that I get when I run the function

Invalid UpdateExpression: Incorrect number of operands for operator or function; operator or function: list_append, number of operands: 1

Data structure in DynamodB

Below is the code:

var caseId       = "1734009";
var chatMessage = { "2018-04-20T15:02:48Z":
                    {
                      "userId": "wQnUJrklzwWBDOsx83XVETSS7us2",
                      "message": "How are you"
                    }
                  }

var params = {
  TableName  : 'CHATS',
  Key: {
    "CASE_ID" : caseId
  },
  UpdateExpression : "SET CHAT_MESSAGES = list_append(:i)",
  ExpressionAttributeValues : {
      ':i': [chatMessage],
  },
  ReturnValues:'UPDATED_NEW' // OTHER OPTIONS: NONE | ALL_OLD | UPDATED_OLD | ALL_NEW | UPDATED_NEW
};


documentClient.update(params, function(err, data) {
  if(err) {

    var message  = "Chat message could not be saved, error:" + JSON.stringify(err, null, 2);
    res.json({"status": "failure", "statusCode" : 400, "message": message});

  } else {
    next();
  }
});

Jason Livesay · Accepted Answer · 2018-04-29 00:23:08Z

1

list_append takes two arguments, not one: https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.UpdateExpressions.html

answered Apr 29, 2018 at 0:23

Jason Livesay

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kurrodu · Accepted Answer · 2018-04-30 00:08:43Z

1

By using the below params and referring to the AWS Docs provided by Jason Livesay, I was able to add items to an empty list.

var params = {
    TableName  : 'CHATS',
    Key: {
      "CASE_ID" : caseId
    },

    UpdateExpression : "SET #ri = list_append(#ri, :vals)",
    ExpressionAttributeNames: {
      "#ri": "CHAT_MESSAGES"
    },
    ExpressionAttributeValues : {
        ':vals': [chatMessage]
    },
    ReturnValues:'UPDATED_NEW' // OTHER OPTIONS: NONE | ALL_OLD | UPDATED_OLD | ALL_NEW | UPDATED_NEW
  };

answered Apr 30, 2018 at 0:08

kurrodu

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Comments

Shreya Rajput · Accepted Answer · 2019-11-27 00:57:20Z

Look at the following example. Attribute 'graph' is created as empty list which can be later updated using list_append(). Please note I am using Boto3(python) to update my dynamodb table.

dynamoTable = dynamodb.Table('abc')
            dynamoTable.put_item(
                Item={
                    ''<primary_key>'': <primary_key_value>,
                    'First Name': 'first_name',
                    'Last Name': 'last_name',
                    'password_hash': 'password_hash',
                    'salt': 'salt',
                    'graph': [],
                }

Look at the following code to append element/message to the list :

dynamoTable = dynamodb.Table('abc')
        dynamoTable.update_item(
        Key={
        '<primary_key>': '<primary_key_value>',
        },
        UpdateExpression="set #Graph = list_append(#Graph, :name)",
        ExpressionAttributeNames={
            '#Graph': 'graph',

     },
        ExpressionAttributeValues = {
                ':name': [# {'myObject':
                    {
                       "userId": "wQnUJrklzwWBDOsx83XVETSS7us2",
                       "message": "How are you"
                    }
                #}
            ],


   }

The code above will append a map in 'graph' with the "userId" and "message"

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