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Amazon web services – How to merge StepFunction ResultSelector with ResultPath?

In AWS, I have a Step Function Workflow:

                    "GetAccountAlias": {
                      "Type": "Task",
                      "Resource": "arn:aws:states:::aws-sdk:iam:listAccountAliases",
                      "Parameters": {
                        "MaxItems": 1
                      },
                      "ResultSelector": {
                        "name.$": "States.ArrayGetItem($.AccountAliases, 0)"
                      },
                      "ResultPath": "$.name",
                      "Next": "Update"
                    },

It gets an input like this:

{
  "my_object_key": "123"
}

Next I retrieve the alias. Now the output is:

{
  "my_object_key": "123",
  "name": {
    "name": "<account-alias>"
  }
}

When I update "ResultPath": "$.name" to "ResultPath": "$" then the output is:

  {
    "name": "<account-alias>"
  }

What I want is:

{
  "my_object_key": "123",
  "name": "<account-alias>"
}

How can I achieve this? (Preferably without using an additional task/stage)

Tags:

2

Answers


  1. 0

    Unfortunately, you cannot accomplish this within the same state as the input is not available in the ResultSelector. You need to do this in a subsequent Pass state (see example below).

      "Format Output": {
      "Type": "Pass",
      "Parameters": {
        "my_object_key.$": "$.my_object_key",
        "name.$": "$.name.name"
      },
      "Next": "My Next State"
    }
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  2. 0

    Yes, if my_object_key is in the *execution* input. If not, you’ll need a Pass state, as @JustinCallison says.

    ResultSelector can reference (1) the lambda task output (as $) and (2) the step function’s execution input (as $$.Execution.Input). If my_object_key was passed into the step function, you are in luck:

    "ResultSelector": {
  "merged.$": "States.JsonMerge(States.StringToJson(States.Format('\{"name":"{}"\}', States.ArrayGetItem($.AccountAliases, 0))), States.StringToJson(States.Format('\{"my_object_key":"{}"\}', $$.Execution.Input.my_object_key)), false)"
},
"OutputPath": "$.merged"

    What’s happening? We’re merging two JSON objects with the States.JsonMerge intrinsic function. Easy enough, but first we must coerce name and my_object_key from values to JSON key-value pairs. We manually create a stringified object with States.Format and then parse it with States.StringToJson.

    Here’s ResultSelector formatted to make it easier to see what’s going on:

    States.JsonMerge(
    States.StringToJson(
        States.Format('\{"name":"{}"\}', States.ArrayGetItem($.AccountAliases, 0))
    ),
    States.StringToJson(
        States.Format('\{"my_object_key":"{}"\}', $$.Execution.Input.my_object_key)
    ),
    false
)
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