Requirement:

• AWS API Gateway has a AWS Lambda integration.
• Append custom data to input json via mapping template.
• Event json sent to Lambda should be simple json (Not a nested json).
• Please Note: Nothing wrong in passing nested json to Integrated Lambda. This post discussing about a requirement to pass only simple json to the Lambda.

API Gateway user provided input json.

{
  "name": "John",
  "phoneNumber": "1231231231"
}

Expected json output

after appending Custom data: ("code" : "abcd")

{
  "name": "John",
  "phoneNumber": "1231231231",
  "code": "abcd"
}

Solution option1: (Not meeting requirement)
AWS Console -> API Gateway -> Select a Resource -> Integration Request -> Mapping Templates -> "When thee are no templates defined" -> Content Type: application/json -> provide below template

{
"body": $input.json('$'),
"code": "abcd"
}

AWS Lambda event appears as below:

{
  "body": {
    "name": "John",
    "phoneNumber": "1231231231"
  },
  "code": "abcd"
}

The above json is not meeting with expected json format mentioned in our requirements stated above.

Solution option2: (meets the expectation):
AWS Console -> API Gateway -> Select a Resource -> Integration Request -> Mapping Templates -> "When thee are no templates defined" -> Content Type: application/json -> provide below template

#set($elem = $util.parseJson($input.json('$')))
{
#foreach($elemKey in $elem.keySet())
    "$elemKey": "$elem.get($elemKey)"
    #if($foreach.hasNext),#end
,
"code": "abcd"
}

AWS Lambda event appears as below:

{
  "name": "John",
  "phoneNumber": "1231231231",
  "code": "abcd"
}

Conclusion:

In real scenario the custom data can be output of Lambda authorizer data getting appended to input json and passed on to Integrated AWS Lambda as event.

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