DynamoDB : SET list_append not working using aws sdk - node.js

Question

I need to append a string to a string set in a dynamodb table using the corresponding key. This is the Update expression I use to do updateItem :
var params = {
"TableName" : tableName,
"Key": {
"ID": {
S: "20000"
}
},
"UpdateExpression" : "SET #attrName = list_append(#attrName, :attrValue)",
"ExpressionAttributeNames" : {
"#attrName" : "entries"
},
"ExpressionAttributeValues" : {
":attrValue" : {"SS":["000989"]}
} };
This works when I do updateItem() using the aws cli. But when using aws-sdk in nodejs, I am getting the error:
Invalid UpdateExpression: Incorrect operand type for operator or function; operator or function: list_append, operand type: M\n
Any help?
Thanks

Answer 1

list_append can be read as a "concatenate" operation. You just give it two lists.
"UpdateExpression" : "SET #attrName = list_append(#attrName, :attrValue)",
"ExpressionAttributeNames" : {
"#attrName" : "entries"
},
"ExpressionAttributeValues" : {
":attrValue" : ["000989"]
}
It's worth remembering that lists (and maps) in DynamoDB are not typed and can store arbitrary data.
Side note: Armed with this knowledge, the documentation on appending to the beginning of the list now makes sense:
list_append (operand, operand)
This function 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.

Answer 2

There's an accepted answer on this question which helped me with part of this issue. However, we'll typically want to update lists with additional objects, not strings. For this, I found it useful to avoid using ExpressionAttributeNames if possible.
1) Make sure the value in your item in your DynamoDB table is a list.
2) Make sure you pass in a list of objects (even if you only have one), not a simple object
UpdateExpression: "set pObj.cObj= list_append(pObj.cObj, :obj)",
ExpressionAttributeValues: {
":obj": [
{myObject: {
property1: '',
property2: '',
property3: '',
}}
]
},

Answer 3

I thought I'd just throw this out there as another option for adding or appending an "object" to a list. It's a map being added an item to the list, and worked well for me:
var upsertExpr = (obj.comments == undefined) ? " :attrValue" : "list_append(#attrName, :attrValue)";
var params = {
TableName: 'tableName',
Key: {
'id': {'S': id},
},
UpdateExpression : "SET #attrName = " + upsertExpr,
ExpressionAttributeNames : {
"#attrName" : "comments"
},
ExpressionAttributeValues : {
":attrValue" : {
"L": [
{ "M" :
{
"comment": {"S": comment},
"vote": {"N": vote.toString()}
}
}
]
}
}
};

Answer 4

maybe this will help someone. i was struggling with updating a list and was getting the same error message as the original poster. i managed to solve my problem when i finally understood the documentation (see the Adding Elements To a List example here http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.UpdateExpressions.html#Expressions.UpdateExpressions.ADD)
points to note are: 1) that the "list_append takes two lists as input, and appends the second list to the first." and 2) that ExpressionAttributeValues is a list! like this:
{
":vals": {
"L": [
{ "S": "Screwdriver" },
{"S": "Hacksaw" }
]
}
}
good luck!