Handling Nested Integration response from aws apigateway - json

Question

I have a JSON response from AWS Lambda going to AWS API-GATEWAY as follows :-
[
{
"key1" : "fruit",
"key2" : "citrus",
"key3" : {
"key31" : "lemon",
"key32" : "orange",
"key33" : "lime"
}
},
{
"key1" : "vegetable",
"key2" : "green",
"key3" : {
"key31" : "spinach",
"key32" : "lettuce",
"key33" : "cabbage"
}
}
]
Before sending to the client application from API Gateway, I want to modify the keys in the response as below:
[
{
"category" : "fruit",
"subCategory" : "citrus",
"examples" : {
"eg1" : "lemon",
"eg2" : "orange",
"eg3" : "lime"
}
},
{
"category" : "vegetable",
"subCategory" : "green",
"examples" : {
"eg1" : "spinach",
"eg2" : "lettuce",
"eg33" : "cabbage"
}
}
]
In AWS ApiGateway we have Mapping Templates to transform the response coming from Lambda and going out of API Gateway using the Apache Velocity.
I am using application/json format to create the mapping template.
Below is the code I am written for the transformation --
#set($inputRoot = $input.path('$'))
[
#foreach($elem in $inputRoot)
{
"category": "$elem.key1",
"subCategory": "$elem.key2",
"examples" : #set($example in $elem.key3)
{
"eg1" : "$example.key31",
"eg2" : "$example.key32",
"eg3" : "$example.key33"
}#end
}#if($foreach.hasNext),#end
#end
]
The response which I am receiving from the api gateway after hitting it as below ---
{
"message": "Internal server error"
}
I am still new with the API Gateways so if anyone could help, it would be really great.