get everything working with the engine included and devise routes declared within it.

Gemfile.lock

@@ -23,6 +23,7 @@ PATH
   remote: test/rails_app/rails_engine
   specs:
     rails_engine (0.0.1)
+      devise
       rails
 
 GEM

test/rails_app/rails_engine/app/models/rails_engine/user.rb

@@ -1,5 +1,5 @@
 module RailsEngine
-  class User
-    table_name :user
+  class User < ActiveRecord::Base
+    self.table_name = :user
   end
 end

test/rails_app/rails_engine/config/routes.rb

@@ -1,5 +1,5 @@
 RailsEngine::Engine.routes.draw do
-  # devise_for :engine_users, class_name: RailsEngine::User
+  devise_for :engine_users, class_name: 'RailsEngine::User'
 
   # devise_for :engine_users_with_router_name, router_name: :rails_engine
 end

test/rails_app/rails_engine/lib/rails_engine/engine.rb

@@ -1,3 +1,4 @@
+require 'devise'
 module RailsEngine
   class Engine < ::Rails::Engine
     isolate_namespace RailsEngine

test/rails_app/rails_engine/rails_engine.gemspec

@@ -13,4 +13,5 @@ Gem::Specification.new do |s|
   s.files = Dir["{app,config,db,lib}/**/*", "MIT-LICENSE", "Rakefile"]
 
   s.add_dependency "rails"
+  s.add_dependency "devise"
 end