in memory DB

sqlite.rb

require 'active_record'
require 'sqlite3'


ActiveRecord::Base.establish_connection(
    :adapter => 'sqlite3',
    :database => ':memory:'
  )

ActiveRecord::Base.connection.instance_eval do
  create_table :firstnames, :id => false do |t|
    t.string :firstname
    t.integer :age
  end

  add_index :firstnames, :firstname
end


class Firstnames < ActiveRecord::Base
end

Firstnames.create(firstname: 'john', age: 13)
Firstnames.create(firstname: 'julie', age: 20)
Firstnames.create(firstname: 'bob', age: 15)

p Firstnames.where("age > 5").order('age')