Get one element from module output list(target_group_arns of terraform-aws-modules/alb/aws)

[Anton7K]

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cdktf & Language Versions

cdktf 0.6.3
cdktf-cdktf-provider-aws 2.0.11
terraform-aws-modules/alb/aws@5.16.0
Python 3.8.10

Affected Resource(s)

• aws alb module(TerraformAwsModulesAlbAws - https://registry.terraform.io/modules/terraform-aws-modules/alb/aws/latest)
• EcsService

I created several aws albs via module and want to get single element from their output list (target_group_arns) to create subsequent resources(ecs services).
I tried different approaches, the last one was as following(for test purposes I hardcoded index values):

lb = self.load_balancers[0]
target_group_arn = Fn.element(Token.as_list(lb.target_group_arns_output), 0)  

Expected Behavior

I thought, that in json output will appear something like:
"target_group_arn": "${element(module.stack-name-lb2.target_group_arns, 0)}"
And then this string will be substituted with needed value during apply

Actual Behavior

I can synthesize code successfully, but during the plan I get several errors like this one:

Error: Invalid character
│
│ on cdk.tf.json line 2704, in resource.aws_ecs_service.stack-nameecs-cluster-construct-service-33_EEFE3F15.load_balancer[0]:
│ 2704: "target_group_arn": "${element(${module.stack-name-lb2.target_group_arns}, 0)}"
│
│ This character is not used within the language.
╵
╷
│ Error: Invalid expression
│
│ on cdk.tf.json line 2704, in resource.aws_ecs_service.stack-nameecs-cluster-construct-service-33_EEFE3F15.load_balancer[0]:
│ 2704: "target_group_arn": "${element(${module.stack-name-lb2.target_group_arns}, 0)}"
│
│ Expected the start of an expression, but found an invalid expression token.

[jsteinich]

Related to #287 and #525, but does seem to be a standalone issue.

One workaround:

• Pass lb.fqn + '.target_group_arns' to the element function

[Anton7K]

Thank you very much! It seems like it is working.
But I have one more question: how to deal with nested lists in general and ones in outputs especially?

I divided my configuration in several stacks. One of them has nested list as output. Now I need to get this list from another stack and extract separate elements.

Output variable is in the following format:

nested_output : 
[
	[
		“string…”,
		“string…”
	],
	[	
		“string…”,
		“string…”,
		“string…”,
	],
	... 
] 

When I tried to use nested Fn.element, I got the same error during the plan as previously:

code:

remote_state = DataTerraformRemoteState(
	self,
	"remote_state_1",
	workspaces=NamedRemoteWorkspace(“workspace_name”),
	organization=”org_name”
)
self.nested_output = remote_state.get_list("nested_output")

Fn.element(
	Token.as_list(Fn.element(self.nested_output,0))
	1
)

error:

Error: Invalid character
│
│ on cdk.tf.json line 97, in output.test:
│ 97: "value": "${element(${element(data.terraform_remote_state.remote_state_1.outputs.nested_output, 0)}, 1)}"
│
│ This character is not used within the language.
╵
╷
│ Error: Invalid expression
│
│ on cdk.tf.json line 97, in output.test:
│ 97: "value": "${element(${element(data.terraform_remote_state.remote_state_1.outputs.nested_output, 0)}, 1)}"
│Expected the start of an expression, but found an invalid expression token.

I didn’t find fqn property of DataTerraformRemoteState, so I’m using following solution now:

Fn.element(
              Token.as_list("data.terraform_remote_state.remote_state_1.outputs.nested_output[0]"),
              1)
)

It seems like it is working, but I think it is not the best solution. Are there any other options?

Thank you in advance!

[jsteinich]

It seems like it is working, but I think it is not the best solution. Are there any other options?

Seems like you're hitting a bug, so the workaround you have is fine.

[github-actions]

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