how do I read from a Database in ruby sinatra using active record

I want to read entrys from a Database using active_record and keep getting diffrent Errors like: Name error and can't find the database or it can't execute the query.

so my question here is how do i read from a database or execute SQL-queries and for example write the result into a variable?

require 'rubygems'
require 'sinatra'
require 'active_record'
require 'sqlite3'


ActiveRecord::Base.establish_connection(
  :adapter  => "sqlite3",
  :database => "test.db"
)


class Article < ActiveRecord::Base
end

#ActiveRecord::Migration.create_table :users do |t|
#  t.string :name
#end


class App < Sinatra::Application
end

get '/' do
  testoutput = testi.select(:all)
  f = File.open('name','a'); f.write(output); f.close
  #puts  User.first