Environment: Mongoid v. 4.0.0.rc1. Solves problem when running db:mongoid:create_indexes or any other mongoid rake task doesn't take into account models defined in engines used by your main application.

load_model.rake

# main_app/lib/tasks/load_model.rake
namespace :db do
  namespace :mongoid do

    # include models from engine
    task :load_models do
      Scraper::Engine.config.paths['app/models'].to_a.each do |path|
        preload = ::Mongoid.preload_models
        if preload.resizable?
          files = preload.map { |model| "#{path}/#{model}.rb" }
        else
          files = Dir.glob("#{path}/**/*.rb")
        end

        files.sort.each do |file|
          require_dependency(file)
        end
      end
    end
  end
end

Very nice! Thanks