Insert into existing map a map structure in DynamoDB using Nodejs

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': [],
  }
 };

Mohammed Rashid · Accepted Answer · 2017-01-31 20:21:37Z

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 }

answered Jan 31, 2017 at 20:21

Mohammed Rashid

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1 Comment

Khalid T. Over a year ago

Glad you worked it out :) What I understood from your original question was that you always had a parent #car and you were just trying to add an element to the #model list, which might not exist.

Khalid T. · Accepted Answer · 2017-01-30 12:31:30Z

0

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)

edited Jan 30, 2017 at 12:31

answered Jan 29, 2017 at 18:54

Khalid T.

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4 Comments

Mohammed Rashid Over a year ago

Is there no way to do it in a single statement?

Khalid T. Over a year ago

Not to my knowledge, no.

Mohammed Rashid Over a year ago

I tried this part alone SET #car.#model = if_not_exists(#car.#model, :empty_list) . It doesnt work. Gives error : "The document path provided in the update expression is invalid for update",

Khalid T. Over a year ago

Can you please provide the complete working code (including the values for :empty_list and :value) in order to re-produce the problem?

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