UpdateExpression: Add other attribute's value to list - amazon-web-services

Question

Given the following DynamoDB document:
{
"myobject" : {"foo" : "bar"},
"mylist" : [{"some" : "stuff}]
}
My goal is to update this document to get the following result:
{
"myobject" : {"foo" : "bar"},
"mylist" : [{"some" : "stuff}, {"foo" : "bar"}]
}
My request's params look like this:
let params = {
TableName: doctorSlotsTable,
Key: {
hashKey: hash,
rangeKey: range
},
UpdateExpression: 'SET mylist = list_append(if_not_exists(mylist, :empty_list), [myobject])',
ExpressionAttributeValues : {
':empty_list' : []
},
ReturnValues : "UPDATED_NEW"
};
This obviously does not work because the [ in the list_append triggers a syntax error.
Is there any solution to achieve that without having to get the data in a previous request and add it manually to the list ?

Answer 1

Unfortunately you cannot use an attribute name as an operand to list_append(...) unless that attribute is itself a list. The best you can do I believe would be to store myobject in the proper type up front, and then update it as expected.
Since storage is cheap & network/compute are expensive here, you could even duplicate the data to have one of them in the right form.
Here's a full example, where createTable() and deleteTable() do exactly what you think:
const PK = 'the item';
async function createObjAndList() {
const docClient = new DocumentClient();
const myObject = { foo: "bar" };
const theItem = {
PK,
myObject,
myObjectAsList: [ myObject ],
myList: [ { some : "stuff" } ],
};
const putParams = {
TableName,
Item: theItem
}
await docClient.put(putParams).promise();
console.log(`Put item ${util.inspect(theItem)}`);
}
async function updateListWithObject() {
const docClient = new DocumentClient();
const updateParams = {
TableName,
Key: { PK },
UpdateExpression: `SET #myList = list_append(if_not_exists(#myList, :emptyList), #myObjectAsList)`,
ExpressionAttributeNames: {
'#myList': 'myList',
'#myObjectAsList': 'myObjectAsList',
},
ExpressionAttributeValues: {
':emptyList': [],
}
}
await docClient.update(updateParams).promise();
console.log(`Updated list to include object`);
}
async function getObjAndList() {
const docClient = new DocumentClient();
const results = await docClient.get({ TableName, Key: { PK }}).promise();
console.log(`Item is now: ${util.inspect(results.Item)}`);
}
if (module === require.main) {
(async () => {
try {
await createTable();
await createObjAndList()
await updateListWithObject();
await getObjAndList();
} catch (err) {
console.log(`Error: ${err.message}`);
} finally {
await deleteTable();
}
})();
}
The output from this is:
Put item {
PK: 'the item',
myObject: { foo: 'bar' },
myObjectAsList: [ { foo: 'bar' } ],
myList: [ { some: 'stuff' } ]
}
Updated list to include object
Item is now: {
myList: [ { some: 'stuff' }, { foo: 'bar' } ],
myObject: { foo: 'bar' },
PK: 'the item',
myObjectAsList: [ { foo: 'bar' } ]
}