ActiveRecord trying to save readonly records on has_many_through assignment. It looks like the bug was introduced by https://github.com/rails/rails/commit/d849f42b4ecf687ed5350f5a2402fb795aa33aac, and it should probably check `if !record.readonly?` and not just remove the condition.

gist.rb

require 'bundler/inline'

gemfile(true) do
  source 'https://rubygems.org'
  gem 'activerecord', '5.0.0.beta3'
  gem 'sqlite3'
end

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 :clubs, force: true

  create_table :memberships, force: true do |t|
    t.integer :member_id
    t.integer :club_id
  end

  create_table :members, force: true
end

class Club < ActiveRecord::Base
  has_many :memberships
  has_many :members, through: :memberships
end

class Membership < ActiveRecord::Base
  belongs_to :member
  belongs_to :club
end

class Member < ActiveRecord::Base
  def readonly?
    persisted?
  end
end

class BugTest < Minitest::Test
  def test_assign_attributes
    club = Club.create!
    member = Member.create!
    club.assign_attributes(member_ids: [member.id])

    assert_equal [member], club.members
  end

  def test_collection_shovel
    club = Club.create!
    member = Member.create!
    club.members << member

    assert_equal [member], club.members
  end
end