https://github.com/rails/rails/issues/7490

reproduction.rb

require 'bundler'
Bundler.setup(:default)

require 'active_record'
require 'minitest/autorun'
require 'logger'

# 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 :things do |t|
  end

  create_table :foos do |t|
    t.integer :thing_id
  end

  create_table :bars do |t|
    t.integer :thing_id
  end
end

class Thing < ActiveRecord::Base
  has_many :foos
  has_many :bars

  scope :with_foos, -> { preload(:foos) }
  scope :with_bars, -> { preload(:bars) }
end

class Foo < ActiveRecord::Base
  belongs_to :thing
end

class Bar < ActiveRecord::Base
  belongs_to :thing
end

class BugTest < Minitest::Unit::TestCase
  def test_association_stuff
    assert_equal [:foos, :bars], Thing.with_foos.with_bars.preload_values
  end
end