Automatically guess the inverse associations for STI

ActiveRecord associations automatically guess the inverse associations.
But this feature does not work correctly on assoctions for STI.
For example, before this commit

```
class Post < ActiveRecord::Base
  belongs_to :author
end

class SpecialPost < Post; end

class Author < ActiveRecord::Base
  has_many :posts
  has_many :special_posts
end
```

`author.posts.first.author` works correctly, but
`author.special_posts.first.author` does not work correctly.

3 files changed, 43 insertions, 2 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index b39d28ca31..4a840a57b5 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,7 @@
+*   Automatically guess the inverse associations for STI.
+
+    *Yuichiro Kaneko*
+
 *   Ensure `sum` honors `distinct` on `has_many :through` associations
 
     Fixes #16791.
diff --git a/activerecord/lib/active_record/reflection.rb b/activerecord/lib/active_record/reflection.rb
index b847933b2e..e35049bb41 100644
--- a/activerecord/lib/active_record/reflection.rb
+++ b/activerecord/lib/active_record/reflection.rb
@@ -657,7 +657,7 @@ module ActiveRecord
         # from calling +klass+, +reflection+ will already be set to false.
         def valid_inverse_reflection?(reflection)
           reflection &&
-            klass.name == reflection.active_record.name &&
+            klass <= reflection.active_record &&
             can_find_inverse_of_automatically?(reflection)
         end
 
diff --git a/activerecord/test/cases/associations/inverse_associations_test.rb b/activerecord/test/cases/associations/inverse_associations_test.rb
index 536077fb92..86a034b429 100644
--- a/activerecord/test/cases/associations/inverse_associations_test.rb
+++ b/activerecord/test/cases/associations/inverse_associations_test.rb
@@ -18,6 +18,8 @@ require "models/admin/user"
 require "models/developer"
 require "models/company"
 require "models/project"
+require "models/author"
+require "models/post"
 
 class AutomaticInverseFindingTests < ActiveRecord::TestCase
   fixtures :ratings, :comments, :cars
@@ -60,6 +62,20 @@ class AutomaticInverseFindingTests < ActiveRecord::TestCase
     assert_equal rating_reflection, comment_reflection.inverse_of, "The Comment reflection's inverse should be the Rating reflection"
   end
 
+  def test_has_many_and_belongs_to_should_find_inverse_automatically_for_sti
+    author_reflection = Author.reflect_on_association(:posts)
+    author_child_reflection = Author.reflect_on_association(:special_posts)
+    post_reflection = Post.reflect_on_association(:author)
+
+    assert_respond_to author_reflection, :has_inverse?
+    assert author_reflection.has_inverse?, "The Author reflection should have an inverse"
+    assert_equal post_reflection, author_reflection.inverse_of, "The Author reflection's inverse should be the Post reflection"
+
+    assert_respond_to author_child_reflection, :has_inverse?
+    assert author_child_reflection.has_inverse?, "The Author reflection should have an inverse"
+    assert_equal post_reflection, author_child_reflection.inverse_of, "The Author reflection's inverse should be the Post reflection"
+  end
+
   def test_has_one_and_belongs_to_automatic_inverse_shares_objects
     car = Car.first
     bulb = Bulb.create!(car: car)
@@ -267,7 +283,7 @@ class InverseHasOneTests < ActiveRecord::TestCase
 end
 
 class InverseHasManyTests < ActiveRecord::TestCase
-  fixtures :men, :interests
+  fixtures :men, :interests, :posts, :authors
 
   def test_parent_instance_should_be_shared_with_every_child_on_find
     m = men(:gordon)
@@ -281,6 +297,27 @@ class InverseHasManyTests < ActiveRecord::TestCase
     end
   end
 
+  def test_parent_instance_should_be_shared_with_every_child_on_find_for_sti
+    a = authors(:david)
+    ps = a.posts
+    ps.each do |p|
+      assert_equal a.name, p.author.name, "Name of man should be the same before changes to parent instance"
+      a.name = "Bongo"
+      assert_equal a.name, p.author.name, "Name of man should be the same after changes to parent instance"
+      p.author.name = "Mungo"
+      assert_equal a.name, p.author.name, "Name of man should be the same after changes to child-owned instance"
+    end
+
+    sps = a.special_posts
+    sps.each do |sp|
+      assert_equal a.name, sp.author.name, "Name of man should be the same before changes to parent instance"
+      a.name = "Bongo"
+      assert_equal a.name, sp.author.name, "Name of man should be the same after changes to parent instance"
+      sp.author.name = "Mungo"
+      assert_equal a.name, sp.author.name, "Name of man should be the same after changes to child-owned instance"
+    end
+  end
+
   def test_parent_instance_should_be_shared_with_eager_loaded_children
     m = Man.all.merge!(where: { name: "Gordon" }, includes: :interests).first
     is = m.interests