Add regression test for has_many_through associations failing to create join models

This commit adds a test that demonstrates a regression in has_many_through associations
where multiple has_many_through associations associate the same parent with the same
child relation.

The regression is ultimately caused by rails/rails@0d38f2a, which avoids saving a through association target
if we expect the target to be persisted by another (direct) association.

The problem is that if we have multiple has_many_through associations that associate the same parent with the same
child, we will autosave the first set of associations from the parent, but avoid saving the through target a second
time (once it's persisted by the first association), and fail to create the join model for the second association chain.

Author: adrianna-chang-shopify

activerecord/test/cases/associations/has_many_through_associations_test.rb

@@ -29,6 +29,9 @@
 require "models/member"
 require "models/membership"
 require "models/club"
+require "models/program"
+require "models/program_offering"
+require "models/enrollment"
 require "models/organization"
 require "models/user"
 require "models/family"
@@ -1548,6 +1551,21 @@ def test_has_many_through_update_ids_with_conditions
     assert_equal [], author.nonspecial_categories_with_condition_ids
   end
 
+  def test_has_many_through_from_same_parent_to_same_child_creates_join_models
+    club = Club.new(name: "Awesome Rails Club")
+    member = club.simple_members.build(name: "Jane Doe")
+
+    program = Program.new(name: "Learn Ruby on Rails")
+    program.members << member
+
+    club.programs << program
+
+    club.save!
+
+    assert_equal(1, program.enrollments.size)
+    assert_equal(1, club.simple_memberships.size)
+  end
+
   def test_single_has_many_through_association_with_unpersisted_parent_instance
     post_with_single_has_many_through = Class.new(Post) do
       def self.name; "PostWithSingleHasManyThrough"; end

activerecord/test/models/club.rb

@@ -15,6 +15,12 @@ class Club < ActiveRecord::Base
 
   scope :general, -> { left_joins(:category).where(categories: { name: "General" }).unscope(:limit) }
 
+  has_many :program_offerings
+  has_many :programs, through: :program_offerings
+
+  has_many :simple_memberships, class_name: "Membership"
+  has_many :simple_members, through: :simple_memberships, foreign_key: "member_id"
+
   accepts_nested_attributes_for :membership
 
   private

activerecord/test/models/enrollment.rb

@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+
+class Enrollment < ActiveRecord::Base
+  belongs_to :program
+  belongs_to :member, class_name: "SimpleMember"
+end

activerecord/test/models/member.rb

@@ -44,6 +44,13 @@ class Member < ActiveRecord::Base
   scope :with_member_type_id, -> (id) { where(member_type_id: id) }
 end
 
+class SimpleMember < ActiveRecord::Base
+  self.table_name = "members"
+
+  has_many :memberships
+  has_many :clubs, through: :memberships
+end
+
 class SelfMember < ActiveRecord::Base
   self.table_name = "members"
   has_and_belongs_to_many :friends, class_name: "SelfMember", join_table: "member_friends"

activerecord/test/models/membership.rb

@@ -4,6 +4,9 @@ class Membership < ActiveRecord::Base
   enum :type, %i(Membership CurrentMembership SuperMembership SelectedMembership TenantMembership)
   belongs_to :member
   belongs_to :club
+  has_one :sponsor, through: :club
+
+  belongs_to :simple_member, foreign_key: "member_id"
 end
 
 class CurrentMembership < Membership

activerecord/test/models/program.rb

@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+
+class Program < ActiveRecord::Base
+  has_many :enrollments
+  has_many :members, through: :enrollments, class_name: "SimpleMember"
+end

activerecord/test/models/program_offering.rb

@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+
+class ProgramOffering < ActiveRecord::Base
+  belongs_to :club
+  belongs_to :program
+end

activerecord/test/schema/schema.rb

@@ -588,6 +588,11 @@
     t.references :car, index: false
   end
 
+  create_table :enrollments, force: true do |t|
+    t.integer  :program_id
+    t.integer  :member_id
+  end
+
   create_table :entrants, force: true do |t|
     t.string  :name, null: false
     t.integer :course_id, null: false
@@ -1018,6 +1023,16 @@
     t.decimal :discounted_price
   end
 
+  create_table :program_offerings, force: true do |t|
+    t.integer  :club_id
+    t.integer  :program_id
+    t.datetime :start_date
+  end
+
+  create_table :programs, force: true do |t|
+    t.string   :name
+  end
+
   add_check_constraint :products, "price > discounted_price", name: "products_price_check"
 
   create_table :product_types, force: true do |t|