Rails with Shoulda and multiple scopes uniques fails tests

Question

What am I doing wrong here? I'm pretty sure the actual code is right but there's something wrong with my tests Is this just a fail in Shoulda Matchers? Or am I just missing something?

require 'rails_helper'

RSpec.describe Game, type: :model do
  subject { create :game }

  it { should belong_to(:home_team).class_name('Team') }
  it { should belong_to(:away_team).class_name('Team') }

  it { should validate_uniqueness_of(:start_date).scoped_to(:home_team_id) }
  it { should validate_uniqueness_of(:start_date).scoped_to(:away_team_id) }
end

class Game < ActiveRecord::Base
  belongs_to :home_team, class_name: 'Team'
  belongs_to :away_team, class_name: 'Team'

  validates_uniqueness_of :start_date, scope: :home_team_id
  validates_uniqueness_of :start_date, scope: :away_team_id
end

   Expected Game to validate that :start_date is case-sensitively unique
   within the scope of :home_team_id, but this could not be proved.
     Given an existing Game whose :start_date is ‹Fri, 31 Aug 2018 05:24:25
     UTC +00:00›, after making a new Game and setting its :start_date to
     ‹Fri, 31 Aug 2018 05:24:25 UTC +00:00› as well and its :home_team_id
     to a different value, ‹5›, the matcher expected the new Game to be
     valid, but it was invalid instead, producing these validation errors:

     * start_date: ["has already been taken"]

Answer 1

Sorry, my previous answer was incorrect. Looks like shoulda can't hadle your case automatically. It changes home_team_id and expects that the game is valid, but it still has the same away_team_id, so you need to validate it manually

it 'validates uniqueness for both teams' do
  team1 = create :team
  team2 = create :team
  team3 = create :team
  game = create :game, home_team: team1, away_team: team2
  game1 = build :game, home_team: team1, away_team: team3
  game2 = build :game, home_team: team3, away_team: team2
  game3 = build :game, home_team: team2, away_team: team1

  expect(game).to be_valid
  expect(game1).not_to be_valid
  expect(game2).not_to be_valid
  expect(game3).to be_valid
end