Storing optional attributes in DynamoDB's putItem via step functions

Question

I have defined a state machine in AWS step functions and one of my states is storing an item to DynamoDB

...
"Store item": {
  "End": true,
  "Type": "Task",
  "Resource": "arn:aws:states:::dynamodb:putItem",
  "Parameters": {
    "Item": {
      "foo": {
        "S.$": "$.data.foo"
      },
      "bar": {
        "S.$": "$.data.bar"
      },
      "baz": {
        "S.$": "$.data.baz"
      },
    },
    "TableName": "nrp_items"
  }
},
...

The problem starts from the fact that baz property is optional, ie not exist in some cases. On those cases, the putItem task fails:

An error occurred while executing the state 'Store item' (entered at the event id #71). > The JSONPath '$.data.baz' specified for the field 'S.$' could not be found in the input

My backup plan is to use a lambda to perform that type of operation, but can I do it directly using the putItem task in steps function?

I was wondering if:

1. Is possible to somehow inject via JSONPath my whole $.data item to the "Item" property, something like:

...
"Store item": {
  "End": true,
  "Type": "Task",
  "Resource": "arn:aws:states:::dynamodb:putItem",
  "Parameters": {
    "Item": "$.data",
    "TableName": "nrp_items"
  }
},
...

OR
2) Define that the baz property is optional

Answer 1

TL;DR We can deal with optional variables with a "Variable": "$.baz", "IsPresent": true Choice condition to handle no-baz cases.

The Amazon States Language spec does not have optional properties: Step Functions will throw an error if $.baz does not exist in the input. We can avoid undefined paths by inserting a two-branch Choice State, one branch of which handles baz-exists cases, the other no-baz cases. Each branch continues with a Pass State that reworks the data input into dynamo-format Item syntax, using Parameters. The put-item task's "Item.$": "$.data" (as in your #1) contains only foo-bar when baz is not defined, but all three otherwise.

{
  "StartAt": "HasBazChoice",
  "States": {
    "HasBazChoice": {
      "Type": "Choice",
      "Choices": [
        {
          "Variable": "$.baz",
          "IsPresent": true,
          "Next": "MakeHasBazItem"
        }
      ],
      "Default": "MakeNoBazItem"
    },
    "MakeHasBazItem": {
      "Type": "Pass",
      "Parameters": {
        "data": {
          "foo": { "S.$": "$.foo"},
          "bar": { "S.$": "$.bar"},
          "baz": { "S.$": "$.baz"}
        }
      },
      "Next": "PutItemTask"
    },
    "MakeNoBazItem": {
      "Type": "Pass",
      "Parameters": {
        "data": {
          "foo": {"S.$": "$.foo"},
          "bar": {"S.$": "$.bar"}
        }
      },
      "Next": "PutItemTask"
    },
    "PutItemTask": {
      ...
      "Parameters": {
        "TableName": "my-table",
        "Item.$": "$.data"
      }
    },
  }
}

If you have more than one optional field, your lambda backup plan is the better option - the above workaround would become unwieldy.