ActiveRecord: #merge with joins and a has_many :through creates redundant INNER JOINs and postgresql does not like those

ar_merge_joins_hmt.rb

system 'rm Gemfile' if File.exist?('Gemfile')
File.write('Gemfile', <<-GEMFILE)
  source 'https://rubygems.org'
  gem 'activerecord',
    '5.0.1'
    # '4.2.0'
    # '4.1.8'
    # '4.0.12'
    # '3.2.21'
  gem 'sqlite3'
GEMFILE

system 'bundle install'

require 'byebug'
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 :companies do |t|
    t.string :name
  end

  create_table :titles do |t|
    t.string :name
    t.boolean :obscene_salary
  end

  create_table :positions do |t|
    t.belongs_to :company
    t.belongs_to :title

    t.string :location
  end

  create_table :jobs do |t|
    t.belongs_to :position

    t.date :started_at
    t.string :full_name
  end
end

class Company < ActiveRecord::Base
  has_many :positions
  has_many :titles,
    through: :positions
end

# think of titles as being regulated, companies can't just create a title
class Title < ActiveRecord::Base
  has_many :positions
  has_many :companies,
    through: :positions

  # titles which had at least one position with at least one active job on a given date
  scope :active_on, -> (date) { joins(:positions).uniq.merge(Position.active_on(date)) }
end

class Position < ActiveRecord::Base
  belongs_to :title
  belongs_to :company

  has_many :jobs

  # positions which had at least one active job on a given date
  scope :active_on, -> (date) { joins(:jobs).uniq.merge(Job.active_on(date)) }
end

class Job < ActiveRecord::Base
  belongs_to :position

  # jobs which were active on a given date
  scope :active_on, -> (date) { where('started_at <= ?', date) }

  def active?(date)
    started_at <= date
  end
end

# class BugTest < Minitest::Test
class BugTest < MiniTest::Unit::TestCase
  def setup
    # One Company
    @acme = Company.create name: 'Acme'

    # Titles
    @cto = Title.create name: 'CTO', obscene_salary: true
    @clerk = Title.create name: 'Clerk', obscene_salary: true

    # Positions
    @kl_cto = Position.create title: @cto, company: @acme, location: "King's Landing"
    @kl_clerk = Position.create title: @clerk, company: @acme, location: "King's Landing"
    @ss_clerk = Position.create title: @clerk, company: @acme, location: "Sunspear"

    # Jobs
    @albert = Job.create position: @kl_cto, started_at: Date.civil(2015, 1, 1), full_name: 'Albert Doe'
    @bruce = Job.create position: @kl_clerk, started_at: Date.civil(2015, 2, 1), full_name: 'Bruce Doe'
    @carl = Job.create position: @ss_clerk, started_at: Date.civil(2015, 3, 1), full_name: 'Carl Doe'
  end

  def test_merge_with_joins
    assert_equal 2, Title.active_on(Date.civil(2015, 3, 10)).count # this is sometimes 4 ¯\_(ツ)_/¯
    assert_equal 1, Title.active_on(Date.civil(2015, 1, 10)).count
  end

  def test_merge_with_joins_and_has_many
    assert_equal 2, @acme.titles.active_on(Date.civil(2015, 3, 10)).count
    assert_equal 1, @acme.titles.active_on(Date.civil(2015, 1, 10)).count

    sql = @acme.titles.active_on(Date.civil(2015, 3, 10)).to_sql
    puts sql
    assert_equal 1,
      sql.scan(/INNER JOIN .positions/).count,
      'hmt and explicit joins duplicate the INNER JOIN on positions, which Postgres does not approve'
  end
end

Actually, a more appropriate description would be that postgresql does not like the reference to "positions" in the second join ("positions"."id") when that is defined later. Query:

SELECT DISTINCT "titles".* 
  FROM "titles"
    INNER JOIN "positions" "positions_titles"
      ON "positions_titles"."title_id" = "titles"."id"
    LEFT OUTER JOIN "jobs"
      ON "jobs"."position_id" = "positions"."id"
    INNER JOIN "positions"
      ON "titles"."id" = "positions"."title_id"
  WHERE "positions"."company_id" = 2
    AND (started_at <= '2015-03-10')

This is what Postgresql says with this query:

invalid reference to FROM-clause entry for table "positions"
HINT:  Perhaps you meant to reference the table alias "positions_titles".