Determine velocity template request body property type? - amazon-web-services

Question

I have an API Gateway that uses velocity templates as a thin wrapper to allow users to do CRUD operations on a DynamoDB table.
I'm trying to write the update operation as dynamically as possible, but where I'm stuck is with determining type from the request body's properties from within the velocity template. This is what I'm working with:
#set($body = $input.path('$'))
#set($updateExpression = "set")
#set($expressionAttributeNames = "")
#set($expressionAttributeValues = "")
#foreach($attrName in $body.keySet())
#set($updateExpression = "${updateExpression} #$attrName = :${attrName},")
#set($expressionAttributeNames = "${expressionAttributeNames}""#${attrName}"":""${attrName}""")
#set($attrValue = $input.json("$.${attrName}"))
#if($attrValue.matches("^-?\\d+$"))
#set($attrValue = """:${attrName}"": { ""N"": ${attrValue}, ")
#else
#set($attrValue = """:${attrName}"": { ""S"": """ + $util.escapeJavaScript($attrValue) + """ },")
#end
#set($expressionAttributeValues = "${expressionAttributeValues} ${attrValue}")
#if($foreach.hasNext)
#set($expressionAttributeNames = "${expressionAttributeNames}, ")
#end
#end
{
"TableName": "TABLE",
"Key": { "id": { "S": "$input.params('id')" } },
"UpdateExpression": "${updateExpression} updatedOn = :updatedOn",
"ExpressionAttributeNames": {$expressionAttributeNames},
"ExpressionAttributeValues": {
$expressionAttributeValues
":updatedOn": { "N": "$context.requestTimeEpoch" }
}
}
Edit: This would be a sample request body:
https://api/v1/endpoint/123
{
"location": {
"lat": 42,
"lon": -71
},
"rating": 4
}
This is the current transformation I get:
{
"TableName": "users",
"Key": { "gcn": { "S": "123" } },
"UpdateExpression": "set #number = :number, #location = :location, updatedOn = :updatedOn",
"ExpressionAttributeNames": {"#number":"number", "#location":"location"},
"ExpressionAttributeValues": {
":number": { "S": "1" }, ":location": { "S": "{\"lat\":26.89199858375187,\"lon\":75.77141155196833}" },
":updatedOn": { "N": "" }
}
}
I currently just have a test for checking if a value is a number...and it isn't working.

Answer 1

After doing some more digging I reached what I set out for. I have a dynamic Velocity Template mapping for AWS API Gateway for the purpose of updating DynamoDB items.
So far it supports strings, numbers, booleans, and string-escaped objects, as that's how my project stores them (they are not query-able). ExpressionAttributeNames exists in case you use a reserved keyword for an attribute name...like I did for 'location'.
If anyone has any improvements/enhancements please let me know, it's a beast of a script.
#set($body = $input.path('$'))
#set($updateExpression = "set")
#set($expressionAttributeNames = "")
#set($expressionAttributeValues = "")
#foreach($attrName in $body.keySet())
#set($updateExpression = "${updateExpression} #$attrName = :${attrName},")
#set($expressionAttributeNames = "${expressionAttributeNames}""#${attrName}"":""${attrName}""")
#set($attrValue = $input.json("$.${attrName}"))
#if($attrValue.toString().matches("[+-]?\d+"))
#set($attrValue = """:${attrName}"": { ""N"": ""${attrValue}"" }, ")
#elseif($attrValue.toString() == "true" || $attrValue.toString() == "false")
#set($attrValue = """:${attrName}"": { ""BOOL"": ${attrValue} }, ")
#elseif(($attrValue.toString().startsWith("{") && $attrValue.toString().endsWith("}")) ||
($attrValue.toString().startsWith("[") && $attrValue.toString().endsWith("]")) )
#set($attrValue = """:${attrName}"": { ""S"": """ + $util.escapeJavaScript($attrValue) + """ },")
#else
#set($attrValue = """:${attrName}"": { ""S"": " + $attrValue + " },")
#end
#set($expressionAttributeValues = "${expressionAttributeValues} ${attrValue}")
#if($foreach.hasNext)
#set($expressionAttributeNames = "${expressionAttributeNames}, ")
#end
#end
{
"TableName": "", ## Insert your table here.
"Key": { "gcn": { "S": "$input.params('')" } }, ## Insert your key expression here.
## Update below if `updatedOn` is not your audit attribute.
"UpdateExpression": "${updateExpression} updatedOn = :updatedOn",
"ExpressionAttributeNames": {$expressionAttributeNames},
"ExpressionAttributeValues": {
$expressionAttributeValues
":updatedOn": { "N": "$context.requestTimeEpoch.toString()" }
}
}
Sample Request Body:
{
"firstName": "John",
"isActive": true,
"_status": 1
}
Sample Transformation:
{
"TableName": "users",
"Key": {
"id": {
"S": "1"
}
},
"UpdateExpression": "set #firstName = :firstName, #isActive = :isActive, #_status = :_status, updatedOn = :updatedOn",
"ExpressionAttributeNames": {
"#firstName": "firstName",
"#isActive": "isActive",
"#_status": "_status"
},
"ExpressionAttributeValues": {
":firstName": {
"S": "John"
},
":isActive": {
"BOOL": true
},
":_status": {
"N": "1"
},
":updatedOn": {
"N": "123456789"
}
}
}