Is it possible to pass an array (of objects) as an input payload to External Call REST API Transformation in Dataflow?

Question

I am trying to send an array of objects in below format to externalcall to a RESTAPI

[{"abc":"v1","cde":"value2"},{"abc":"v4","cde":"v5"},{"abc":"v7","cde":"v8"}]

However I encounter 2 issues in Externalcall transformation step

1. The list of objects within the array is taken as a string. How can it be changed to array of objects?

Below is the data copied from Copy request payload

Uri: https://xxx.sdfdgfg.xxx/xxx/rest/xxx/test

Method: POST

Headers: content-type: application/json | Authorization: ******

Body: {"req_array":["{"abc":"v1","cde":"v2"},{"abc":"v4","cde":"v5"},{"abc":"v7","cde":"v8"}"]}

2. The request body being sent always prefix the columnname (req_array) within the request, due to which I get unknown JSON keyword encountered error response from API.

Below sample error response.

{"status":"error","reason":"Unknown JSON keyword encountered. Valid keywords for the test request are abc and cde."}

Answer 1

Hi @Anantharaman Rajagopal

The array is currently being treated as a string. Because the square brackets inside req_array indicate that the array is being treated as a string rather than a JSON array. To fix this, use the toJSON() function in a Derived Column transformation before passing the data to the External Call transformation.
In the Derived Column transformation, apply:

toJSON(yourColumnName)

This ensures the column retains its JSON structure instead of being converted to a string.

The Flatten transformation in ADF Data Flow to break the array into individual rows, ensuring each object is processed separately. Then, apply the Aggregate transformation to regroup the data into a JSON array without adding a column name, preserving the correct API payload structure.
The below Microsoft docs will be helpful for reference

https://learn.microsoft.com/en-us/azure/data-factory/data-flow-expression-functions#tojson
https://learn.microsoft.com/en-us/azure/data-factory/data-flow-aggregate
https://learn.microsoft.com/en-us/azure/data-factory/data-flow-flatten
https://www.youtube.com/watch?v=PieRpstpRJg
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