Reproduce the preloader regression in #41596

It's very peculiar, what seem to happen is that if some associations
are already loaded, the batching is done properly.

activerecord/test/cases/associations_test.rb

@@ -27,6 +27,10 @@ require "models/parrot"
 require "models/bird"
 require "models/treasure"
 require "models/price_estimate"
+require "models/invoice"
+require "models/discount"
+require "models/line_item"
+require "models/shipping_line"
 
 class AssociationsTest < ActiveRecord::TestCase
   fixtures :accounts, :companies, :developers, :projects, :developers_projects,
@@ -435,6 +439,60 @@ class PreloaderTest < ActiveRecord::TestCase
     end
   end
 
+  def test_some_already_loaded_associations
+    item_discount = Discount.create(amount: 5)
+    shipping_discount = Discount.create(amount: 20)
+
+    invoice = Invoice.new
+    line_item = LineItem.new(amount: 20)
+    line_item.discount_applications << LineItemDiscountApplication.new(discount: item_discount)
+    invoice.line_items << line_item
+
+    shipping_line = ShippingLine.new(amount: 50)
+    shipping_line.discount_applications << ShippingLineDiscountApplication.new(discount: shipping_discount)
+    invoice.shipping_lines << shipping_line
+
+    invoice.save!
+    invoice.reload
+
+    # SELECT "line_items".* FROM "line_items" WHERE "line_items"."invoice_id" = ?
+    # SELECT "shipping_lines".* FROM shipping_lines WHERE "shipping_lines"."invoice_id" = ?
+    # SELECT "line_item_discount_applications".* FROM "line_item_discount_applications" WHERE "line_item_discount_applications"."line_item_id" = ?
+    # SELECT "shipping_line_discount_applications".* FROM "shipping_line_discount_applications" WHERE "shipping_line_discount_applications"."shipping_line_id" = ?
+    # SELECT "discounts".* FROM "discounts" WHERE "discounts"."id" IN (?, ?).
+    assert_queries(5) do
+      preloader = ActiveRecord::Associations::Preloader.new(records: [invoice], associations: [
+        line_items: { discount_applications: :discount },
+        shipping_lines: { discount_applications: :discount },
+      ])
+      preloader.call
+    end
+
+    assert_no_queries do
+      assert_not_nil invoice.line_items.first.discount_applications.first.discount
+      assert_not_nil invoice.shipping_lines.first.discount_applications.first.discount
+    end
+
+    invoice.reload
+    invoice.line_items.map { |i| i.discount_applications.to_a }
+    # `line_items` and `line_item_discount_applications` are already preloaded, so we expect:
+    # SELECT "shipping_lines".* FROM shipping_lines WHERE "shipping_lines"."invoice_id" = ?
+    # SELECT "shipping_line_discount_applications".* FROM "shipping_line_discount_applications" WHERE "shipping_line_discount_applications"."shipping_line_id" = ?
+    # SELECT "discounts".* FROM "discounts" WHERE "discounts"."id" = ?.
+    assert_queries(3) do
+      preloader = ActiveRecord::Associations::Preloader.new(records: [invoice], associations: [
+        line_items: { discount_applications: :discount },
+        shipping_lines: { discount_applications: :discount },
+      ])
+      preloader.call
+    end
+
+    assert_no_queries do
+      assert_not_nil invoice.line_items.first.discount_applications.first.discount
+      assert_not_nil invoice.shipping_lines.first.discount_applications.first.discount
+    end
+  end
+
   def test_preload_through
     comments = [
       comments(:eager_sti_on_associations_s_comment1),

activerecord/test/models/discount.rb

@@ -0,0 +1,4 @@
+# frozen_string_literal: true
+
+class Discount < ActiveRecord::Base
+end

activerecord/test/models/invoice.rb

@@ -2,5 +2,6 @@
 
 class Invoice < ActiveRecord::Base
   has_many :line_items, autosave: true
+  has_many :shipping_lines, -> { from("shipping_lines") }, autosave: true
   before_save { |record| record.balance = record.line_items.map(&:amount).sum }
 end

activerecord/test/models/line_item.rb

@@ -2,4 +2,10 @@
 
 class LineItem < ActiveRecord::Base
   belongs_to :invoice, touch: true
+  has_many :discount_applications, class_name: "LineItemDiscountApplication"
+end
+
+class LineItemDiscountApplication < ActiveRecord::Base
+  belongs_to :line_item
+  belongs_to :discount
 end

activerecord/test/models/shipping_line.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class ShippingLine < ActiveRecord::Base
+  belongs_to :invoice, touch: true
+  has_many :discount_applications, class_name: "ShippingLineDiscountApplication"
+end
+
+class ShippingLineDiscountApplication < ActiveRecord::Base
+  belongs_to :shipping_line
+  belongs_to :discount
+end

activerecord/test/schema/schema.rb

@@ -313,6 +313,10 @@ ActiveRecord::Schema.define do
     t.boolean :deleted
   end
 
+  create_table :discounts, force: true do |t|
+    t.integer :amount
+  end
+
   create_table :dl_keyed_belongs_tos, force: true, id: false do |t|
     t.primary_key :belongs_key
     t.references :destroy_async_parent
@@ -547,6 +551,11 @@ ActiveRecord::Schema.define do
     t.integer :amount
   end
 
+  create_table :line_item_discount_applications, force: true do |t|
+    t.integer :line_item_id
+    t.integer :discount_id
+  end
+
   create_table :lions, force: true do |t|
     t.integer :gender
     t.boolean :is_vegetarian, default: false
@@ -922,6 +931,16 @@ ActiveRecord::Schema.define do
     t.integer :shape_id
   end
 
+  create_table :shipping_lines, force: true do |t|
+    t.integer :invoice_id
+    t.integer :amount
+  end
+
+  create_table :shipping_line_discount_applications, force: true do |t|
+    t.integer :shipping_line_id
+    t.integer :discount_id
+  end
+
   create_table :ships, force: true do |t|
     t.string :name
     t.integer :pirate_id