Mounted engine URLs are missing subdomain from constraint

[lukeredpath]

Steps to reproduce

Given an isolated engine called MyEngine, a namespaced controller called Foo::DashboardsController and the following routes:

constraints(subdomain: 'admin') do
  namespace: 'foo', path: 'portal' do
    root to: 'dashboards#index'
    mount MyEngine::Engine => '/my_engine'
  end
end

The MyEngine engine has a single WidgetsController and it's routes look like this:

root to: 'widgets#index'
resources :widgets

When trying to use URL helpers from outside a controller/view (e.g. in a service class or a job, on the console or anything outside of a request context), any generated URLs for my engine routes do not contain the subdomain while the routes for the main app do.

Expected behavior

From within a view, the following URL helpers output the following results:

foo_root_url => 'http://admin.example.com/portal'
main_app.foo_root_url => 'http://admin.example.com/portal'
foo_my_engine.root_url => 'http://admin.example.com/portal/my_engine'
foo_my_engine.widgets_url => 'http://admin.example.com/portal/my_engine/widgets'

All of these are what I would expect - the subdomain is included in the default URL options for anything within this constraint, the path set on the namespace is included and the routing helpers (including the engine helper object) require prefixing with the namespace to work.

Actual behavior

The problem occurs when trying to generate a URL for my engine, with the same output above, but say within a service class or a background job. Using the Rails console, I can generate the URLs for the main app as expected:

Rails.application.routes.url_helpers.foo_root_url => 'http://admin.example.com/portal'

However, foo_my_engine.root_url does not work:

Rails.application.routes.url_helpers.foo_my_engine.root_urll => NoMethodError: undefined method `foo_my_engine'

It seems I need to use #mounted_helpers to get access to the engine routes, however this is where the problem lies, with both the main_app and the engine helpers

include Rails.application.routes.mounted_helpers
main_app.foo_root_url => 'http://example.com/portal'
foo_my_engine.root_url => 'http://example.com/portal/my_engine'

I would expect these to output the same values that I get in my view, with the admin. subdomain included.

There may be an API I am missing but I've spent time digging through the Rails source to try and understand why I'm getting different results on the console (and in my service classes where I'm trying to actually generate the URLs) than in the view.

System configuration

Rails version: 5.2.1
Ruby version: ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-darwin17]

[rails-bot]

This issue has been automatically marked as stale because it has not been commented on for at least three months.
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