AWS SAM FindInMap Not Populating Variable

Question

I am trying to get a simple SAM template to populate environmental variables "dynamically" using the !FindInMap intrinsic function. I have followed many examples, including AWS's documentation, without any luck. For some reason the function will not populate environment variables using it even though everything seems to be correct. It will just set the variable to an empty string.

You can see from the code below that I am using a !Ref function inside of it, but have tried hardcoding the parameters of the function without any luck. You'll also notice that the function is in the Global section, and you may think it's not working because it's there and not function environmentals, but I've tried both with neither of them working. You'll also notice that I am populating a environment variable called STAGE which is working correctly and setting it to "local".

I am testing the function by running sam start local-api and outputting the environment variables in the response.

Any suggestions would be very helpful.

AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31
Description: "Test Server"

Parameters:
  Environment:
    Type: String
    Default: local
    AllowedValues:
      - local
      - test
      - prod

Mappings:
  EnvParams:
    local:
      stage: "local"
      databaseUrl: "mongodb://localhost:32768/test"

Globals:
    Function:
        Timeout: 500
        Runtime: nodejs8.10
        Environment:
          Variables:
            STAGE: !Ref Environment
            DB_URL: !FindInMap [EnvParams, !Ref Environment, databaseUrl]

Resources:
    ArticlesGetFunction:
        Type: AWS::Serverless::Function # More info about Function Resource: https://github.com/awslabs/serverless-application-model/blob/master/versions/2016-10-31.md#awsserverlessfunction
        Properties:
            CodeUri: src/articles/
            Handler: index.getById
            Events:
                HelloWorld:
                    Type: Api # More info about API Event Source: https://github.com/awslabs/serverless-application-model/blob/master/versions/2016-10-31.md#api
                    Properties:
                        Path: /api/article/
                        Method: get

Outputs:
    HelloWorldApi:
      Description: "API Gateway endpoint URL for Prod stage for Hello World function"
      Value: !Sub "https://${ServerlessRestApi}.execute-api.${AWS::Region}.amazonaws.com/Prod/hello/"

    HelloWorldFunction:
      Description: "Hello World Lambda Function ARN"
      Value: !GetAtt HelloWorldFunction.Arn

    HelloWorldFunctionIamRole:
      Description: "Implicit IAM Role created for Hello World function"
      Value: !GetAtt HelloWorldFunctionRole.Arn

Answer 1

It looks like !FindInMap isn't supported in local debugging yet. Here's the relevant GitHub issue: https://github.com/awslabs/aws-sam-cli/issues/476

To set and test Environment Variables in SAM CLI, you can use the --env-vars option instead. !FindInMap is also supported when deployed via CloudFormation, you could test this feature by deploying a simple Lambda function and running a test query against it.