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One of the major changes in Rails is the use of zeitwerk as the code loader. Zeitwerk will warn if any unsafe loading happens in initializers. The solution to this problem is usually to wrap any code which autoloads classes in a Rails.application.config.to_prepare block. For examples see https://github.com/avalonmediasystem/avalon/pull/4657.
Another change is a requirement to allow list hosts that the rails server will respond to. Avalon by default will allow the host set in the `domain.host` setting as well as any set in the RAILS_ADDITIONAL_HOSTS environment variable. The docker-compose config (both the development environment in avalon and the avalon-docker environment) allows the avalon host which docker creates for its internal network. If you connect to your avalon instance using a different hostname then these, you will probably need to set RAILS_ADDITIONAL_HOSTS environment variable or make changes to config/initializers/default_host.rb.
Blacklight
Blacklight 7 changes include the upgrade of bootstrap to 4, changes to search builders, and switching the /catalog json endpoints to return standard JSON-API. If you have customized the CatalogController, any of the search builders, or any of the catalog views, you may need to make changes to ensure compatibility. See the Blacklight 7 release notes for more details: https://github.com/projectblacklight/blacklight/releases/tag/v7.0.0
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