Why does this AWS API Gateway Mapping Template not output the correct variable? - velocity

Question

I am using AWS API Gateway. I have a response from my backend and I am trying to map the response into a different output using a Mapping Template.
I am trying to return each of the hits.hits._source.display elements. But the mapping template returns the entire object with the literal "_source.display" string added to the end.
The original unmodified server response is like -
{
"took":7,
"timed_out":false,
"_shards":{
"total":1,
"successful":1,
"skipped":0,
"failed":0
},
"hits":{
"total":{
"value":10000,
"relation":"gte"
},
"max_score":1.0,
"hits":[
{
"_index":"address",
"_type":"_doc",
"_id":"GAVIC411064535",
"_score":1.0,
"_source":{
"id":"GAVIC411064535",
"display":"1/28 George Street, Casterton VIC 3311",
"location":{
"lat":-37.59205672,
"lon":141.38825665
},
"component":{
"buildingName":"",
"number":"1/28",
"street":"George Street",
"locality":"Casterton",
"state":"VIC",
"postcode":"3311"
}
}
},
{
"_index":"address",
"_type":"_doc",
"_id":"GAVIC411066597",
"_score":1.0,
"_source":{
"id":"GAVIC411066597",
"display":"52 St Georges Road, Corio VIC 3214",
"location":{
"lat":-37.59205672,
"lon":141.38825665
},
"component":{
"buildingName":"",
"number":"52",
"street":"St Georges Road",
"locality":"Corio ",
"state":"VIC",
"postcode":"3214"
}
}
},
My Mapping Template so far looks like this -
#set($inputRoot = $input.path('$'))
{
#foreach($elem in $inputRoot.hits.hits)
{
"address": $elem._source.display,
}#if($foreach.hasNext),#end
#end
}
The resulting output looks like this. You can see that its returing the entire $elem rather than returning the properties I've asked for. That is ._source.display
It has also literally printed ._source.display to the end.
{
"address": {_index=address, _type=_doc, _id=GAVIC411064535, _score=1.0, _source={id=GAVIC411064535, display=1/28 George Street, Casterton VIC 3311, location={lat=-37.59205672, lon=141.38825665}, component={buildingName=, number=1/28, street=George Street, locality=Casterton, state=VIC, postcode=3311}}}._source.display,
},
{
"address": {_index=address, _type=_doc, _id=GAVIC411066597, _score=1.0, _source={id=GAVIC411066597, display=52 St Georges Road, Corio VIC 3214, location={lat=-37.59205672, lon=141.38825665}, component={buildingName=, number=52, street=Georges Road, locality=Corio , state=VIC, postcode=3214}}}._source.display,
},
I have truncated the server response and mapping template output for brevity.
The desired mapping template output is -
{
"address" : "28 George Street, Casterton VIC 3311",
"address" : "52 St Georges Road, Corio VIC 3214"
}

Answer 1

This is a bug in version 1.7 which has been fixed in 2.x versions (in July 2016... AWS really ought to upgrade its libraries sometimes).
You can work around this bug like this:
#foreach($elem in $inputRoot.hits.hits)
{
"address": "$elem.get('_source').display"
}#if($foreach.hasNext),#end
#end