A basic sinatra example

accounts.rb

require 'sinatra'
require 'json'
require_relative './database.rb'

get '/accounts' do
  content_type :json
  begin
    @user = User.find(params[:user_id])
    {:accounts => @user.accounts.map(&:attributes)}.to_json
  rescue ActiveRecord::RecordNotFound => e
    [].to_json
  end
end

database.rb

require 'sinatra'
require 'sinatra/activerecord'

set :database, 'sqlite:///db/gateway.db'

class User < ActiveRecord::Base
  has_many :accounts
  
  def full_name
    [first_name, last_name].compact.join(" ")
  end
end

class Account < ActiveRecord::Base
  belongs_to :user
end

Rakefile

require_relative './database'
require 'sinatra/activerecord/rake'