active_record_master.rb

unless File.exist?('Gemfile')
  File.write('Gemfile', <<-GEMFILE)
    source 'https://rubygems.org'
    gem 'rails', github: 'rails/rails'
    gem 'arel', github: 'rails/arel'
    gem 'sqlite3'
  GEMFILE

  system 'bundle'
end

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 :posts do |t|
    t.boolean :trashed
  end

  create_table :comments do |t|
    t.boolean :trashed
    t.integer :post_id
  end
end

class Post < ActiveRecord::Base
  has_many :comments, -> { unscope(where: :trashed) }

  default_scope -> { where(trashed: false) }
end

class Comment < ActiveRecord::Base
  belongs_to :post, -> { unscope(where: :trashed) }

  default_scope -> { where(trashed: false) }
end

class BugTest < Minitest::Test
  def test_includes_has_many_scope
    post = Post.create!
    comment = Comment.create!(trashed: true)
    post.comments << comment

    assert_equal comment, Post.first.comments.first
    # Pass
    # SELECT  "comments".* FROM "comments"  WHERE "comments"."post_id" = ?  ORDER BY "comments"."id" ASC LIMIT 1  [["post_id", 1]]


    assert_equal comment, Post.includes(:comments).first.comments.first
    # Failure
    # SELECT "comments".* FROM "comments"  WHERE "comments"."trashed" = 'f' AND "comments"."post_id" IN (1)
  end

  def test_includes_belongs_to_scope
    post = Post.create!(trashed: true)
    comment = Comment.create!
    post.comments << comment

    assert_equal post, Comment.first.post
    # Pass
    # SELECT  "posts".* FROM "posts"  WHERE "posts"."id" = ? LIMIT 1  [["id", 1]]

    assert_equal post, Comment.includes(:post).first.post
    # Failure
    # SELECT "posts".* FROM "posts"  WHERE "posts"."trashed" = 'f' AND "posts"."id" IN (1)
  end
end