How can I create or update a map using update expression? - amazon-web-services

Question

I have a scenario where I want to create an item if it doesn't exist, or update an item - incrementing a total, if it already exists.
I was running into problems splitting the two operations, so I am now trying to do both using UpdateItem in a single command.
I've tried 3 different approaches none work, and they have different errors listed below, the problem it seems is creating the map and trying to update it in a single command - what should my update params look like?
Attempt one:
{
TableName: TableName,
Key: {
'key': key
},
UpdateExpression: `
ADD #total :change
, mapname.#type.#total :one
`,
ExpressionAttributeValues: {
':change': change,
':one': 1
},
ExpressionAttributeNames: {
'#type': 'dynamicstring',
'#total': 'total'
}
};
With an error of: ValidationException: The document path provided in the update expression is invalid for update
Attempt two:
{
TableName: TableName,
Key: {
"key": key
},
UpdateExpression: `
SET custommap = if_not_exists(custommap, :emptyMap)
SET #total = #total + :change,
custommap.#type.#total = custommap.#type.#total + :one
`,
ExpressionAttributeValues: {
':change': change,
':one': 1,
':emptyMap': {
'M': {
'dynamicstring': {
'M': {
'total': {
'N': 0
}
}
}
}
}
},
ExpressionAttributeNames: {
'#type': 'dynamicstring',
'#total': 'total'
}
}
With an error of: ValidationException: Invalid UpdateExpression: The "SET" section can only be used once in an update expression;
So when I use UpdateItem to create or update (increment) a map within an Item, what syntax is correct?
Thanks

Answer 1

SET will only stop you overwriting an attribute, not an item.
They way to achieve this is:
Use GetItem with your key to see if the item already exists
If the item exists, then do an UpdateItem and increment the counter
If the item does not exist, then use PutItem