Clear cached has_one association after setting belongs_to association to nil

Given an author that "has one" post, and setting the post's author to nil,
the author will still have a cached association to the post. Updating any
attribute of the author would erroneously restore the post's author. This
commit resets the association, preventing that behavior.

activerecord/CHANGELOG.md

@@ -1,3 +1,12 @@
+*   Clear cached `has_one` association after setting `belongs_to` association to `nil`.
+
+    After setting a `belongs_to` relation to `nil` and updating an unrelated attribute on the owner,
+    the owner should still return `nil` on the `has_one` relation.
+
+    Fixes #42597.
+
+    *Michiel de Mare*
+
 *   OpenSSL constants are now used for Digest computations.
 
     *Dirkjan Bussink*

activerecord/lib/active_record/associations/belongs_to_association.rb

@@ -77,6 +77,8 @@ def replace(record)
             raise_on_type_mismatch!(record)
             set_inverse_instance(record)
             @updated = true
+          elsif target
+            remove_inverse_instance(target)
           end
 
           replace_keys(record, force: true)

activerecord/test/cases/associations/belongs_to_associations_test.rb

@@ -1205,6 +1205,19 @@ def test_reassigning_the_parent_id_updates_the_object
     assert_equal companies(:another_firm), client.firm_with_condition
   end
 
+  def test_clearing_an_association_clears_the_associations_inverse
+    author = Author.create(name: "Jimmy Tolkien")
+    post = author.create_post(title: "The silly medallion", body: "")
+    assert_equal post, author.post
+    assert_equal author, post.author
+
+    author.update!(post: nil)
+    assert_nil author.post
+
+    post.update!(title: "The Silmarillion")
+    assert_nil author.post
+  end
+
   def test_destroying_child_with_unloaded_parent_and_foreign_key_and_touch_is_possible_with_has_many_inversing
     with_has_many_inversing do
       book     = Book.create!

activerecord/test/cases/associations/has_one_associations_test.rb

@@ -307,6 +307,19 @@ def test_create_association
     assert_equal account, firm.reload.account
   end
 
+  def test_clearing_an_association_clears_the_associations_inverse
+    author = Author.create(name: "Jimmy Tolkien")
+    post = author.create_post(title: "The silly medallion", body: "")
+    assert_equal post, author.post
+    assert_equal author, post.author
+
+    post.update!(author: nil)
+    assert_nil post.author
+
+    author.update!(name: "J.R.R. Tolkien")
+    assert_nil post.author
+  end
+
   def test_create_association_with_bang
     firm = Firm.create(name: "GlobalMegaCorp")
     account = firm.create_account!(credit_limit: 1000)