Adding a key/value pair to an object in VTL (for 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.

It is so junk, that AWS adopted VTL to deal with json objects or for any type of mapping. — Kannaiyan, Dec 15 '18 at 22:51

score 6 · Answer 1 · answered Apr 09 '19 at 11:11

6

Something like this has worked for me:

#set($body = $input.path('$'))
#set($body.href = "https://example.tld/thingy/$body.id")
$input.json('$')

answered Apr 09 '19 at 11:11

Pablo López Martínez

221
2
3

this is the answer - does exactly what the original question asked for – Tarek Apr 02 '21 at 20:48

score 2 · Answer 2 · answered Aug 16 '18 at 15:19

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.

score 2 · Answer 3 · answered Feb 08 '19 at 16:29

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
}

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

3 Answers3

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