Infer :inverse_of for has_many ..., through:

Closes [rails#49574][]

Issue rails#49574 outlines how an Active Record join model accessed through a
`has_many ..., through:` association is unable to infer an appropriate
`:inverse_of` association by pluralizing a predictably pluralizable
class name.

This commit resolves that issue by also checking a model's reflections
for a pluralized inverse name in addition to whatever's provided through
the `:as` option or inferred in the singular.

The issue shares a code sample:

```ruby
ActiveRecord::Schema.define do
  create_table "listings", force: :cascade do |t|
    t.bigint "list_id", null: false
    t.bigint "pin_id", null: false
  end

  create_table "lists", force: :cascade do |t|
  end

  create_table "pins", force: :cascade do |t|
  end
end

class Pin < ActiveRecord::Base
  has_many :listings
end

class List < ActiveRecord::Base
  has_many :listings
  has_many :pins, through: :listings
end

class Listing < ActiveRecord::Base
  belongs_to :list
  belongs_to :pin
end

class BugTest < Minitest::Test
  def test_association_stuff
    list = List.create!
    pin = list.pins.new

    pin.save!

    assert_equal [pin], list.reload.pins
    assert_equal 1, list.reload.pins.count
  end
end
```

Unfortunately, there isn't a one-to-one mapping in the test suite's
`test/model` directory for this type of structure. The most similar
associations were between the `Book`, `Subscription`, and `Subscriber`.
For the sake of ease, this commit wraps the test block in a new
`skipping_validations_for` helper method to ignore validations declared
on the `Subscription` join table model.

[rails#49574]: rails#49574

Author: seanpdoyle

activerecord/lib/active_record/reflection.rb

@@ -708,9 +708,10 @@ def inverse_name
         def automatic_inverse_of
           if can_find_inverse_of_automatically?(self)
             inverse_name = ActiveSupport::Inflector.underscore(options[:as] || active_record.name.demodulize).to_sym
+            plural_inverse_name = ActiveSupport::Inflector.pluralize(inverse_name)
 
             begin
-              reflection = klass._reflect_on_association(inverse_name)
+              reflection = klass._reflect_on_association(inverse_name) || klass._reflect_on_association(plural_inverse_name)
             rescue NameError => error
               raise unless error.name.to_s == class_name
 
@@ -720,7 +721,7 @@ def automatic_inverse_of
             end
 
             if valid_inverse_reflection?(reflection)
-              inverse_name
+              reflection.name
             end
           end
         end

activerecord/test/cases/associations/has_one_through_disable_joins_associations_test.rb

@@ -8,6 +8,7 @@
 require "models/developer"
 require "models/company"
 require "models/computer"
+require "models/contract"
 require "models/club"
 require "models/membership"
 

activerecord/test/cases/associations/inverse_associations_test.rb

@@ -24,6 +24,7 @@
 require "models/room"
 require "models/contract"
 require "models/subscription"
+require "models/subscriber"
 require "models/book"
 require "models/branch"
 require "models/cpk"
@@ -199,6 +200,16 @@ def test_has_many_and_belongs_to_automatic_inverse_shares_objects_on_comment
     assert_equal comment.body, rating.comment.body, "Changing the original Comment's body should change the Comment's body on the association"
   end
 
+  def test_belongs_to_should_find_inverse_has_many_automatically
+    book = Book.create!
+    subscriber = book.subscribers.new nick: "Nickname"
+
+    subscriber.save!
+
+    assert_equal [subscriber], book.reload.subscribers
+    assert_equal 1, book.reload.subscribers.count
+  end
+
   def test_polymorphic_and_has_many_through_relationships_should_not_have_inverses
     sponsor_reflection = Sponsor.reflect_on_association(:sponsorable)