database.rb

require 'active_record'

ActiveRecord::Base.configurations = YAML.load_file('./database.yml')
ActiveRecord::Base.establish_connection(:development)

class User << ActiveRecord::Base
end

database.yml

development: 
  adapter: "sqlite"
  database: "dev.db"

migration.rb

require './database.rb'

ActiveRecord::Migration.drop_table :users

ActiveRecord::Migration.create_table :users do |t|
  t.string :username
  t.integer :age
  t.datetime :birthday
end

sinatra_main.rb

require 'sinatra'
require 'sinatra/reloader'
require './database.rb'

set :bind, '0.0.0.0'

get '/' do
  users = User.all
  slim :index
end