gistfile1.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 :t1s do |t|
  end
  
  create_table :t2s do |t|
    t.integer :t1_id
  end
  
  create_table :t3s do |t|
    t.integer :t2_id
  end

  create_table :t4s do |t|
    t.integer :t3_id
  end

  create_table :t5s do |t|
    t.integer :t4_id
  end
end

class T1 < ActiveRecord::Base
end

class T2 < ActiveRecord::Base
  belongs_to :t1
end

class T3 < ActiveRecord::Base
  belongs_to :t2
end

class T4 < ActiveRecord::Base
  belongs_to :t3
end

class T5 < ActiveRecord::Base
  belongs_to :t4
end

class BugTest < Minitest::Test
  def test_merge_joins

    t1 = T1.create
    
    rel1 = T1.where(id: t1.id)
    assert_equal(rel1.to_sql, 'SELECT "t1s".* FROM "t1s" WHERE "t1s"."id" = 1')

    rel2 = T2.joins(:t1).merge(rel1)
    puts rel2.to_sql    
    assert(rel2.to_sql.index('FROM "t2s"') < rel2.to_sql.index('JOIN "t1s"'))
    
    rel3 = T3.joins(:t2).merge(rel2)
    puts rel3.to_sql    
    assert(rel3.to_sql.index('JOIN "t2s"') < rel3.to_sql.index('JOIN "t1s"'))
    
    rel4 = T4.joins(:t3).merge(rel3)
    puts rel4.to_sql    
    assert(rel4.to_sql.index('JOIN "t3s"') < rel4.to_sql.index('JOIN "t2s"'))
    assert(rel4.to_sql.index('JOIN "t2s"') < rel4.to_sql.index('JOIN "t1s"'))    
    
    # Raises ActiveRecord::ConfigurationError
    rel5 = T5.joins(:t4).merge(rel4)
  end
end