Erroneous validation on has_many :through with existing record

gistfile1.rb

# Activate the gem you are reporting the issue against.
gem 'activerecord', '4.1.7'
require 'active_record'
require 'minitest/autorun'
require 'logger'

# Ensure backward compatibility with Minitest 4
Minitest::Test = MiniTest::Unit::TestCase unless defined?(Minitest::Test)

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :conversations do |t|
  end

  create_table :conversation_users do |t|
    t.integer :conversation_id
    t.integer :user_id
  end

  create_table :users do |t|
    t.string :name
  end

end

class Conversation < ActiveRecord::Base
  has_many :conversation_users
  has_many :users, through: :conversation_users
end

class ConversationUser < ActiveRecord::Base
  belongs_to :conversation
  belongs_to :user
end

class User < ActiveRecord::Base
  has_many :conversation_users
  has_many :conversations, through: :conversation_users
  validates :name, presence: true
end

class BugTest < Minitest::Test
  def test_conversation_with_valid_existing_user
    user = User.create!(name: "North Bieber")
    assert user.persisted?

    conversation = Conversation.new(user_ids: [user.id])
    conversation.save!

    assert_equal 1, conversation.users.count
    assert_equal [user], conversation.users
  end

  def test_conversation_with_invalid_existing_user
    user = User.new()
    user.save!(validate: false)
    assert user.persisted?

    # This test fails, even though Conversation should not be responsible for validating an associated existing record
    conversation = Conversation.new(user_ids: [user.id])
    conversation.save!

    assert_equal 1, conversation.users.count
    assert_equal [user], conversation.users
  end

end