How dynamically add arn cognito to lambda? - aws-lambda

Question

I would like to add the cognito authorizer to my lambda function, but for this I need arn cognito, which is created in the stack coud formation(CognitoUserPool in my resources section). I'm using serverless framework.
part of the serverless.yml file
graphql:
handler: src/lambda-functions/graphql/index.handler
timeout: 30
memorySize: 2048
events:
- http:
path: graphql
method: any
private: true
cors: true
authorizer:
arn:
Fn::Join:
- ''
- - 'arn:aws:cognito-idp:'
- ${self:provider.region}
- ':'
- Ref: AWS::AccountId
- ':userpool/'
- Ref: CognitoUserPool
I am getting an error while deploying the application:
TypeError: functionArn.split is not a function
While debugging, I discovered that the function to which the output from Fn::join should be passed is object:
{"Fn::Join":["",["arn:aws:cognito-idp:","eu-west-1",":",{"Ref":"AWS::AccountId"},":userpool/",{"Ref":"CognitoUserPool"}]]}
And there should be passed to the function already resolve arn for example:
arn:aws:cognito-idp:eu-west-1:XXXXXXXXXX:userpool/eu-west-XXXXXXX
How to force that the output from Fn::join to be computed and pass that value to the arn property?

Answer 1

According to this, giving the authorizer a name lets you use intrinsic functions to refer to the ARN:
graphql:
handler: src/lambda-functions/graphql/index.handler
timeout: 30
memorySize: 2048
events:
- http:
path: graphql
method: any
private: true
cors: true
authorizer:
name: CognitoAuthorizer #supply a unique name here
arn:
!GetAtt CognitoUserPool.Arn