Fix regression in inverse_of on through associations

`inverse_of` on through associations was accidently removed/caused to
stop working in commit f8d2899 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

1 files changed, 2 insertions, 0 deletions

diff --git a/activerecord/test/schema/schema.rb b/activerecord/test/schema/schema.rb
index 4b7272f10a..d334a2740e 100644
--- a/activerecord/test/schema/schema.rb
+++ b/activerecord/test/schema/schema.rb
@@ -252,6 +252,7 @@ ActiveRecord::Schema.define do
     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 @@ ActiveRecord::Schema.define do
   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|