self_link for Jsonapi::V1::ProjectResource.comments(ToMany) could not be generated

This issue is a (choose one):

• Problem/bug report.
• Feature request.
• Request for support. Note: Please try to avoid submitting issues for support requests. Use Gitter instead.

Checklist before submitting:

• I've searched for an existing issue.
• I've asked my question on Gitter and have not received a satisfactory answer.
• I've included a complete bug report template. This step helps us and allows us to see the bug without trying to reproduce the problem from your description. It helps you because you will frequently detect if it's a problem specific to your project.
• The feature I'm asking for is compliant with the JSON:API spec.

Description

I get a bunch of warnings that look like this:

self_link for Jsonapi::V1::ProjectResource.comments(ToMany) could not be generated

but there's no indication of what I've done to cause them or what I could do to resolve it.

Start with the Rails 5 bug-report template linked in the issue and create a minimal reproduction for the warning on Jsonapi::V1::ProjectResource.comments(ToMany). Trace how that relationship's self_link is generated and record the missing context needed to explain the warning. Done means the cause and a concrete resolution are established and covered by an appropriate regression check.