Fix regression in inverse_of on through associations

`inverse_of` on through associations was accidently removed/caused to
stop working in commit f8d2899d which was part of a refactoring on
`ThroughReflection`.

To fix we moved `inverse_of` and `check_validity_of_inverse!` to the
`AbstractReflection` so it's available to the `ThroughReflection`
without having to dup any methods. We then need to delegate `inverse_name`
method in `ThroughReflection`. `inverse_name` can't be moved to
`AbstractReflection` without moving methods that set the instance
variable `@automatic_inverse_of`.

This adds a test that ensures that `inverse_of` on a `ThroughReflection`
returns the correct class name, and the correct record for the inverse
relationship.

Fixes #21692

activerecord/lib/active_record/reflection.rb

@@ -175,6 +175,20 @@ def counter_cache_column
         end
       end
 
+      def inverse_of
+        return unless inverse_name
+
+        @inverse_of ||= klass._reflect_on_association inverse_name
+      end
+
+      def check_validity_of_inverse!
+        unless polymorphic?
+          if has_inverse? && inverse_of.nil?
+            raise InverseOfAssociationNotFoundError.new(self)
+          end
+        end
+      end
+
       # This shit is nasty. We need to avoid the following situation:
       #
       #   * An associated record is deleted via record.destroy
@@ -377,14 +391,6 @@ def check_validity!
         check_validity_of_inverse!
       end
 
-      def check_validity_of_inverse!
-        unless polymorphic?
-          if has_inverse? && inverse_of.nil?
-            raise InverseOfAssociationNotFoundError.new(self)
-          end
-        end
-      end
-
       def check_preloadable!
         return unless scope
 
@@ -436,12 +442,6 @@ def has_inverse?
         inverse_name
       end
 
-      def inverse_of
-        return unless inverse_name
-
-        @inverse_of ||= klass._reflect_on_association inverse_name
-      end
-
       def polymorphic_inverse_of(associated_class)
         if has_inverse?
           if inverse_relationship = associated_class._reflect_on_association(options[:inverse_of])
@@ -924,6 +924,8 @@ def primary_key(klass)
           klass.primary_key || raise(UnknownPrimaryKey.new(klass))
         end
 
+        def inverse_name; delegate_reflection.send(:inverse_name); end
+
       private
         def derive_class_name
           # get the class_name of the belongs_to association of the through reflection

activerecord/test/cases/associations/inverse_associations_test.rb

@@ -13,6 +13,9 @@
 require 'models/admin'
 require 'models/admin/account'
 require 'models/admin/user'
+require 'models/developer'
+require 'models/company'
+require 'models/project'
 
 class AutomaticInverseFindingTests < ActiveRecord::TestCase
   fixtures :ratings, :comments, :cars
@@ -198,6 +201,16 @@ def test_associations_with_no_inverse_of_should_return_nil
     belongs_to_ref = Sponsor.reflect_on_association(:sponsor_club)
     assert_nil belongs_to_ref.inverse_of
   end
+
+  def test_this_inverse_stuff
+    firm = Firm.create!(name: 'Adequate Holdings')
+    Project.create!(name: 'Project 1', firm: firm)
+    Developer.create!(name: 'Gorbypuff', firm: firm)
+
+    new_project = Project.last
+    assert Project.reflect_on_association(:lead_developer).inverse_of.present?, "Expected inverse of to be present"
+    assert new_project.lead_developer.present?, "Expected lead developer to be present on the project"
+  end
 end
 
 class InverseHasOneTests < ActiveRecord::TestCase

activerecord/test/models/company.rb

@@ -86,6 +86,9 @@ class Firm < Company
 
   has_many :association_with_references, -> { references(:foo) }, :class_name => 'Client'
 
+  has_one :lead_developer, class_name: "Developer"
+  has_many :projects
+
   def log
     @log ||= []
   end

activerecord/test/models/developer.rb

@@ -54,6 +54,9 @@ def find_least_recent
   has_many :ratings, through: :comments
   has_one :ship, dependent: :nullify
 
+  belongs_to :firm
+  has_many :contracted_projects, class_name: "Project"
+
   scope :jamises, -> { where(:name => 'Jamis') }
 
   validates_inclusion_of :salary, :in => 50000..200000

activerecord/test/models/project.rb

@@ -11,6 +11,8 @@ class Project < ActiveRecord::Base
                             :before_remove => Proc.new {|o, r| o.developers_log << "before_removing#{r.id}"},
                             :after_remove => Proc.new {|o, r| o.developers_log << "after_removing#{r.id}"}
   has_and_belongs_to_many :well_payed_salary_groups, -> { group("developers.salary").having("SUM(salary) > 10000").select("SUM(salary) as salary") }, :class_name => "Developer"
+  belongs_to :firm
+  has_one :lead_developer, through: :firm, inverse_of: :contracted_projects
 
   attr_accessor :developers_log
   after_initialize :set_developers_log

activerecord/test/schema/schema.rb

@@ -252,6 +252,7 @@ def except(adapter_names_to_exclude)
     t.string   :name
     t.string   :first_name
     t.integer  :salary, default: 70000
+    t.integer :firm_id
     if subsecond_precision_supported?
       t.datetime :created_at, precision: 6
       t.datetime :updated_at, precision: 6
@@ -658,6 +659,7 @@ def except(adapter_names_to_exclude)
   create_table :projects, force: true do |t|
     t.string :name
     t.string :type
+    t.integer :firm_id
   end
 
   create_table :randomly_named_table1, force: true do |t|