sinatra+admin.rb

require 'json'

module MyApp
  class API < Sinatra::Base
    content_type :json
    Report.all.to_json
  end

  class Admin < Sinatra::Base
    use SinatraAdmin::App
    SinatraAdmin.register 'Report'
  end
end


# rackup file
require 'myapp.rb'

map "/api"
  run MyApp::API
end

map "/admin" do #mounted over "/admin" namespace(mandatory)
  run MyApp::Admin
end