title: cannot load models within mountable engine on rails
body: <p>I have a rails project which uses mongo db, and I wrote an mountable engine named 'report_service'.</p>
  
  <p>I used it like this in main rails project:</p>
  
  <p><code>gem 'report_service', :git =&gt; 'git@xx.com:report_service.git', :branch =&gt; :master, :require =&gt; false</code></p>
  
  <p>I don't want this gem loaded when the rails project is initialized, so I added the <code>:require =&gt; false</code> option.</p>
  
  <p>But in my rails console, after I execute <code>require 'report_service'</code>, I cannot find models in this gem.</p>
  
  <pre><code>[1] pry(main)&gt; ReportService
  =&gt; ReportService
  [2] pry(main)&gt; ReportService::Engine
  NameError: uninitialized constant ReportService::Engine
  from (pry):2:in `&lt;main&gt;'
  [3] pry(main)&gt; require 'report_service'
  =&gt; true
  [4] pry(main)&gt; ReportService::Engine
  =&gt; ReportService::Engine
  [5] pry(main)&gt; ReportService::RsExam
  NameError: uninitialized constant ReportService::RsExam
  from (pry):5:in `&lt;main&gt;'
  </code></pre>
  
  <p>Here is my report_service gem directory and code:</p>
  
  <p>report_service/lib/report_service.rb</p>
  
  <pre><code>require "active_record/railtie"
  require "report_service/engine"
  module ReportService
  end
  </code></pre>
  
  <p>report_service/lib/report_service/engine.rb</p>
  
  <pre><code>module ReportService
    class Engine &lt; ::Rails::Engine
     isolate_namespace ReportService
    end
  end
  </code></pre>
  
  <p>report_service/app/models/report_service/rs_exam.rb</p>
  
  <pre><code>module ReportService
    class RsExam &lt; ActiveRecord::Base
    end
  end
  </code></pre>