Insert into existing map a map structure in DynamoDB using Nodejs - json

Question

Structure of an item in database is as shown below:
{
"cars": {
"x": [
{
"time": 1485700907669,
"value": 23
}
]
},
"date": 1483214400000,
"id":"1"
}
I have to add a new item "z" of type list to cars like
{
"cars": {
"x": [
{
"time": 1485700907669,
"value": 23
}
],
"z": [
{
"time": 1485700907669,
"value": 23
}
]
},
"date": 1483214400000,
"id": "1"
}
What would the update expression in Node.js look like if I want to achieve somethings like this?
So far this is what I came up with:
set #car.#model= list_append(if_not_exists(#car.#model, :empty_list), :value)
However, if the item does not exist at the time of creation it throws error. Any idea how to do this?
This is the updated parameter I am using, still doesn't work
var params = {
TableName:table,
Key:{
"id": id,
"date": time.getTime()
},
ReturnValues: 'ALL_NEW',
UpdateExpression: 'SET #car.#model = if_not_exists(#car.#model,
:empty_list)',
ExpressionAttributeNames: {
'#car': 'cars',
'#model':"z"
},
ExpressionAttributeValues: {
':empty_list': [],
}
};

Answer 1

The solution is to update operation in two steps, first create a empty map for the parent since it does not exist in the first place.
So, in my case
SET #car= :empty_map
where :empty_map = {}
after doing this run the other update expression
SET #car.#model = list_append(if_not_exists(#car.#model, :empty_list), :value)
where :empty_list=[] and :value= {
"time": 1485700907669,
"value": 23
}

Answer 2

Break your update expression apart into two separate expressions:
SET #car.#model = if_not_exists(#car.#model, :empty_list) SET #car.#model = list_append(#car.#model, :value)