state.rb

class State < ActiveRecord::Base
  def self.default
    find_by(default: true)
  end

  def make_default!
    PgLock.new(name: 'set_state_default').lock do
      State.update_all(default: false)
      update(default: true)
    end
  end

  def to_s
    name
  end
end

state_spec.rb

require 'rails_helper'

RSpec.describe State, type: :model do
  let!(:state1) {FactoryGirl.create(:state, name: "New", default: false)}
  let!(:state2) {FactoryGirl.create(:state, name: "Closed", default: false)}
  let!(:state3) {FactoryGirl.create(:state, name: "Open", default: true)}

  it '#make_default! sets a default state to true' do
    state1.make_default!

    expect(State.first.default?).to be true
  end

  it '#make_default! sets all other states to false' do
    state1.make_default!

    expect(State.first.default?).to be true
    expect(State.second.default?).to be false
    expect(State.third.default?).to be false
  end
end