MainAppDelegator

thredded.rb

# config/intitializers/thredded.rb

# to allow you to not to have to add `main_app` before every path helper 
# when embedding Thredded within a main-app supplied layout (with navbar and links to the main_app)

# ... Add this to the bottom of the initializer

Rails.application.config.to_prepare do
  Rails.application.reload_routes!
  thredded_methods = (Thredded::Engine.routes.url_helpers.methods + Thredded::UrlsHelper.instance_methods)
    .select { |s| s.to_s.ends_with?("_path", "_url") }
  main_app_delegator = Module.new do
    Rails.application.routes.url_helpers.methods
      .select { |m| m.to_s.ends_with?('_path', '_url') }
      .reject { |m| thredded_path_methods.include?(m).tap { |r| Rails.logger.warn "ignoring conflict: #{m}" if r } }
      .each{ |method_name| send(:define_method, method_name) {|*args| main_app.send(method_name, *args)} }
  end
  ::Thredded::ApplicationController.helper main_app_delegator
end

NB: there is a tempting much simpler solution:

Rails.application.config.to_prepare do
  Rails.application.reload_routes!
  ::Thredded::ApplicationController.helper Rails.application.routes.url_helpers
end

Unfortunately this would define (and redefine) more than just the path and url helpers and would break existing routes in the Engine.

There's an error here. thredded_path_methods doesn't exist it should be thredded_methods.