Rails 6: Zeitwerk::NameError doesn’t load class from module

I experienced this same issue when deploying a Rails 6.0.2 application to an Ubuntu 18.04 server.

Unable to load application: Zeitwerk::NameError: expected file /home/deploy/myapp/app/models/concerns/designation.rb to define constant Designation, but didn’t

I found out that the issue was with zeitwerk. Zeitwerk is the new code loader engine used in Rails 6. It’s meant to be the new default for all Rails 6+ projects replacing the old classic engine. Zeitwerk provides the features of code autoloading, eager loading, and reloading.

Here’s how I solved it:

Navigate to the config/application.rb file on your project.

Add this line within your application module to switch to classic mode for autoloading:

config.autoloader = :classic

Here’s an example:

module MyApp
  class Application < Rails::Application
    # Initialize configuration defaults for originally generated Rails version.
    config.load_defaults 6.0

    # Settings in config/environments/* take precedence over those specified here.
    # Application configuration can go into files in config/initializers
    # -- all .rb files in that directory are automatically loaded after loading
    # the framework and any gems in your application.

    config.autoloader = :classic
  end
end

You can read up more on zeitwerk in this article: Understanding Zeitwerk in Rails 6