defining optional attribute in AWS Api Gateway -> DynamoDB mapping

Question

I'm following along: Using Amazon API Gateway as a proxy for DynamoDB

Taking the blog's example, I want to tighten what and how data is written to a dynamodb table frontend by Api Gateway. To this end I define the following mapping in Api Gateway:

{ 
    "TableName": "Comments",
    "Item": {
      "commentId": {
        "S": "$context.requestId"
       },
       "pageId": {
         "S": "$input.path('$.pageId')"
       },
       "userName": {
         "S": "$input.path('$.userName')"
       },
       "message": {
         "S": "$input.path('$.message')"
       }
    }
}

Following along, testing with the following example works just fine:

{
  "pageId": "breaking-news-story-01-18-2016",
  "userName": "Just Saying Thank You",
  "message": "I really enjoyed this story!!"
}

However, let's say I want to keep the same mapping as above but want to make message optional. How would I do that? I can't get it to work. I've tried:

using the above mapping as is, but sending a body without the message-attribute. --> "One or more parameter values were invalid: An AttributeValue may not contain an empty string" 2 using the above mapping as is, but sending a body with message=null. --> "One or more parameter values were invalid: An AttributeValue may not contain an empty string"
changing above mapping by omitting the definition for mapping -> passing a body now without message succeeds obviously. However, sending a body with message doesn't pass message through (which is what I expected, but wanted to exhaust all options)
Don't use a mapping at all. Obviously that works, but now everything is passed-through unfiltered, which is unwanted.

Obviously I could use AWS lambda instead to do the mapping but this feels like such a common use-case, i.e.: optional attributes, that this must be possible directly in Api Gateway.

Geert-Jan · Accepted Answer · 2016-07-07 16:52:12Z

2

Courtesy of @ka:

Explatation: APi gateway auto-populates non-existing attributes with empty strings or something. This needs to be excluded from the if.

Also notice the critical , right after the if statement.

{ 
    "TableName": "Comments",
    "Item": {
      "commentId": {
        "S": "$context.requestId"
       },
       "pageId": {
         "S": "$input.path('$.pageId')"
       },
       "userName": {
         "S": "$input.path('$.userName')"
       }#if($input.path('$.description') && $input.path('$.message') != ""),
       "message": {
         "S": "$input.path('$.message')"
       }
       #end
    }
}

answered Jul 7, 2016 at 16:52

Geert-Jan

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Comments

Ka Hou Ieong · Accepted Answer · 2016-07-05 21:15:24Z

0

Would you mind trying this mapping template? If the message is NULL, it will not be added to the body.

{ 
    "TableName": "Comments",
    "Item": {
      "commentId": {
        "S": "$context.requestId"
       },
       "pageId": {
         "S": "$input.path('$.pageId')"
       },
       "userName": {
         "S": "$input.path('$.userName')"
       }#if ($.message),
       "message": {
         "S": "$input.path('$.message')"
       }
       #end
    }
}

Thanks, -Ka Hou

answered Jul 5, 2016 at 21:15

Ka Hou Ieong

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2 Comments

Geert-Jan Over a year ago

Sadly that doesn't work: Execution failed due to configuration error: Unable to transform request

Geert-Jan Over a year ago

Got it to work. Thanks for pointing me in the right direction. See my answer

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