Based on the information provided in the extracts, there isn't a direct way to handle a

nil

value for

metadata_public

in the configuration of the

header

mutator. The configuration provided in the extracts doesn't include conditional logic or error handling within the template itself. However, you could potentially handle this at the application level where the headers are being used. If the

X-User-IAM-Public-Metadata

header is

nil

or not present, you could have your application logic handle this case appropriately. For more advanced handling, you might want to consider using the

hydrator

mutator mentioned in the extracts. This mutator allows for fetching additional data from external APIs, which can be then used by other mutators. You could potentially use this to fetch and handle user metadata in a more sophisticated way. For more detailed information, you might want to refer to the Ory Oathkeeper documentation: - [Mutators](https://www.ory.sh/docs/oathkeeper/pipeline/mutator) - [Hydrator Mutator](https://www.ory.sh/docs/oathkeeper/pipeline/mutator#hydrator) - [Header Mutator](https://www.ory.sh/docs/oathkeeper/pipeline/mutator#header)

I'm sorry, but the provided extracts do not contain enough information to provide a detailed example of using a hydrator mutator to retrieve information from Kratos and inject that into the request header. However, I can provide a general idea based on the information available. The

hydrator

mutator works by making an upstream HTTP call to an API specified in the Per-Rule Configuration. The request is a POST request and it contains a JSON representation of the AuthenticationSession struct in the body. The response from the API should be a similar JSON object, but with

extra

or

header

fields modified. Here is a general example of how the

hydrator

mutator could be used:

yaml mutators:   hydrator:     enabled: true     config:       api:         url: <http://my-api/my-endpoint>

In this example, the

hydrator

mutator would make a POST request to

<http://my-api/my-endpoint>

with the AuthenticationSession as the request body. The API at

<http://my-api/my-endpoint>

would need to be set up to handle this request, fetch the necessary information from Kratos, and return a response with the

extra

or

header

fields modified as necessary. For more detailed information and examples, you would need to refer to the [Ory Oathkeeper documentation](https://www.ory.sh/docs/oathkeeper/pipeline/mutator#hydrator).

Thank you for sharing your solution. It appears you're using conditional logic within the Go text/template syntax to check if

metadata_public

and

branch_id

exist before printing the value. This is a great way to handle potential

nil

values and avoid errors. Here's the code you shared for reference:

yaml mutators:   noop:     enabled: true   header:     enabled: true     config:       headers:         X-User-IAM-ID: "{{ print .Subject }}"         X-User-IAM-Branch-Id: "{{ if .Extra.identity.metadata_public }} {{ if .Extra.identity.metadata_public.branch_id }} {{ print .Extra.identity.metadata_public.branch_id }} {{ end }} {{ end }}"

In this configuration, the

X-User-IAM-Branch-Id

header will only be set if

.Extra.identity.metadata_public

and

.Extra.identity.metadata_public.branch_id

exist, preventing potential issues with

nil

values. This is a great example of how flexible the Ory Oathkeeper mutators can be when combined with the power of Go's text/template package.