Adding a key/value pair to an object in VTL (for API Gateway) - aws-api-gateway

Question

I am writing a mapping template for an AWS API Gateway integration response. I would like to add a key/value pair to the JSON object returned my Lambda function.
My function returns some JSON like this:
{
"id": "1234",
"name": "Foo Barstein"
}
I would like the template to output something like this:
{
"id": "1234",
"name": "Foo Barstein",
"href": "https://example.tld/thingy/1234"
}
And my mapping template looks like this:
#set($thingy = $input.json('$'))
#set($thingy.href = "https://example.tld/thingy/$thingy.id")
$thingy
However, my template outputs the unmodified $thingy, without the href I have tried to add.
I've read the VTL user guide, but to no avail.

Answer 1

Something like this has worked for me:
#set($body = $input.path('$'))
#set($body.href = "https://example.tld/thingy/$body.id")
$input.json('$')

Answer 2

There is no easy way to achieve this but you can workaround it:
## Mapping template
#set($body = $input.body)
#set($id = $input.json('$.id'))
{
"custom": {
"href" : "https://example.tld/thingy/$id"
},
"body": $body
}
And then merge all the keys in AWS.Lambda (if you use Lambda):
## Lambda handler
exports.handler = function(event, context) {
const requestParams = Object.assign({}, event.body, event.custom);
// ... function code
}
And requestParams will be what you want.

Answer 3

Following could do the trick. Beware, untested!
{
#set($payload = $util.parseJson($input.json('$')))
#set($body = "{
#foreach ($mapEntry in $payload.entrySet())
""$mapEntry.key"": ""$mapEntry.value"",
#end
""href"": ""$payload.id""
}")
$body
}