When Rails is deployed to a subdirectory per the official guide instructions, JSON:API should take this into account.

Implementation detail: Switched from string concatenation (+) to string interpolation because Rails.application.config.relative_url_root might be nil, which would cause an error as @hlogmans already stated.

Tested with:

• Setting RAILS_RELATIVE_URL_ROOT=/subdirectory
• Adding config.relative_url_root = '/subdirectory' in application.rb or .rb

Fixes #473

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Bug fixes and Changes to Core Features:

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Test Plan:

Unit test: bundle exec ruby -I test test/controllers/controller_test.rb -n test_links_include_relative_root

Manual testing:

1. Add config.relative_url_root = '/subdirectory' to config/application.rb
2. Run Rails server
3. Fetch any resource URL
   Expected result: Link objects contain /subdirectory

Reviewer Checklist:

• Maintains compliance with JSON:API
• Adequate test coverage exists to prevent regressions