With the newer version of rails, application itself is an engine which is the initialized format of abstract class Rails::Application and to access routes of any (host) engine the code is
and for a (mountable) engine the code is
Example:
SomeRailsApplicationName::Application.routes.url_helpers.named_route_path
SomeRailsApplicationName::Application.routes.url_helpers.named_route_path(parameter: 'something')
and for a (mountable) engine the code is
SomeMountableEngineName::Engine.routes.url_helpers.named_route_path
SomeMountableEngineName::Engine.routes.url_helpers.named_route_path(parameter: 'something'
Example:
rails console > Helloworld::Application.routes.url_helpers.about_path
=> "/about"
rails console > BambooUser::Engine.routes.url_helpers.login_path
=> "/users/login"