The correct way to do inheritance in Rails mountable engine

Today, I found a very strange issue while writing a mountable engine using Rails 3. I have the following ApplicationController in my engine:

module Marketplace
  class ApplicationController < ::ApplicationController
    before_filter :merge_abilities

    layout 'marketplace/application'

    def marketplace_current_user
      Marketplace::User.find(current_user.id)
    end

    private

    def merge_abilities
      current_ability.merge(Ability.new(current_user))
    end
  end
end

And my User model definition is

module Marketplace
  class User < ::User
    devise omniauth_providers: [:facebook, :paypal]

    has_one :payout_identity, class_name: "Marketplace::PayoutIdentity"
    has_many :course_purchases, class_name: "Marketplace::CoursePurchase"

    def has_verified_payout_identity?
      self.payout_identity and self.payout_identity.receiver_id
    end
  end
end

After starting up the rails server, the first request to load a page will have the controller run the marketplace_current_user method correctly and load the engine's User class. However any request after the first one will given a strange NameError - "uninitialized constant Marketplace::Marketplace::User".

I tried removing the namespace in marketplace_current_user definition but it will load the main app's User class instead.

At last when I change my ApplicationController to look like this:

class Marketplace::ApplicationController < ::ApplicationController
  before_filter :merge_abilities

  layout 'marketplace/application'

  def marketplace_current_user
    Marketplace::User.find(current_user.id)
  end

  private

  def merge_abilities
    current_ability.merge(Ability.new(current_user))
  end
end

Everything would work fine.

Could someone enlighten me where I got it wrong at the beginning? Was it wrong to do inheritance that way?