app.rb

require 'sinatra'
require 'sinatra/activerecord'
require './config/environments'

get "/" do
  puts "hallo"
end

environments.rb

require 'yaml'

configure :development, :production do
  set :public_folder, Proc.new { File.join root, "public/dist" }
end

configure :development do
  puts "development"
  ActiveRecord::Base.establish_connection(
    adapter: :sqlite3,
    database: "db/development.sqlite3"
  )
end

configure :production do
  puts "production"
  ActiveRecord::Base.establish_connection(
    adapter: :sqlite3,
    database: "db/production.sqlite3"
  )
end

configure :test do
  puts "test"
  ActiveRecord::Base.establish_connection(
    adapter: :sqlite3,
    database: "db/test.sqlite3"
  )
end