Make inversed association stale after updating id

Problem
---

Prior to this commit an inversed association would always return false
for `stale_target?`. This could cause problems when updating ids. For
example, assuming that the `Post#comments` association has an inverse to
the `Comment#post` association:

```rb
comment = post.comments.first

comment.update!(post_id: some_other_post_id)

comment.post should now return the post with some_other_post_id, but
since it was inversed it doesn't go stale and the test fails
refute_equal post, comment.post
```

This commit adds a test to catch this example, as well as a test for
counter caches (we already test that this counter cache behavior works
when `inverse_of` is not set, but when `inverse_of` was set we'd
increment and decrement the counter on the same target, effectively a
no-op).

Solution
---

This commit updates the `stale_target?` method so it no longer checks
whether the association is inversed. That is enough to get the two new
tests passing.

The `@inversed` check was originally added in
https://github.com/rails/rails/pull/9499, but the test from that PR
still passes when we remove `@inversed` because of
https://github.com/rails/rails/pull/31214. Rather than tracking
`@inversed`, we set the inverse when autosaving the has one association,
updating the `@stale_state` in the process.

Removing `@inversed` broke some other tests, however:

  - test_belongs_to_with_touch_option_on_destroy_with_destroyed_parent
  - test_touch_later_an_association_dont_autosave_parent
  - test_counter_caches_are_updated_in_memory_when_the_default_value_is_nil
  - test_counters_are_updated_both_in_memory_and_in_the_database_on_create

All of these followed the same pattern of building a record, setting an
inverse to the new record, and then saving. For example:

```rb
invoice = Invoice.create!(line_items: [line_item])
```

First Rails builds the invoice. Then it sets that new invoice as the
inverse for the `line_item`s `#invoice` association (i.e. so
`line_item.invoice` is the same object as `invoice`). Setting the
inverse also involves updating `@stale_state`, but since it's a new
invoice that will be `nil`. Then everything gets saved, at which point
the `nil` `@stale_state` no longer matches `stale_state`.

Rather than relying on `@inversed` to fix this, instead this commit
takes a similar approach to https://github.com/rails/rails/pull/31214
(setting the inverse when autosaving has_one associations) and sets the
inverse when autosaving has_many associations.

activerecord/lib/active_record/associations/association.rb

@@ -52,7 +52,6 @@ module ActiveRecord
         @loaded = false
         @target = nil
         @stale_state = nil
-        @inversed = false
       end
 
       def reset_negative_cache # :nodoc:
@@ -78,7 +77,6 @@ module ActiveRecord
       def loaded!
         @loaded = true
         @stale_state = stale_state
-        @inversed = false
       end
 
       # The target is stale if the target no longer points to the record(s) that the
@@ -88,7 +86,7 @@ module ActiveRecord
       #
       # Note that if the target has not been loaded, it is not considered stale.
       def stale_target?
-        !@inversed && loaded? && @stale_state != stale_state
+        loaded? && @stale_state != stale_state
       end
 
       # Sets the target of this association to <tt>\target</tt>, and the \loaded flag to +true+.
@@ -137,15 +135,11 @@ module ActiveRecord
 
       def inversed_from(record)
         self.target = record
-        @inversed = !!record
       end
 
       def inversed_from_queries(record)
         if inversable?(record)
           self.target = record
-          @inversed = true
-        else
-          @inversed = false
         end
       end
 

activerecord/lib/active_record/autosave_association.rb

@@ -404,6 +404,8 @@ module ActiveRecord
               saved = true
 
               if autosave != false && (new_record_before_save || record.new_record?)
+                association.set_inverse_instance(record)
+
                 if autosave
                   saved = association.insert_record(record, false)
                 elsif !reflection.nested?
@@ -447,9 +449,7 @@ module ActiveRecord
             if (autosave && record.changed_for_autosave?) || record_changed?(reflection, record, key)
               unless reflection.through_reflection
                 record[reflection.foreign_key] = key
-                if inverse_reflection = reflection.inverse_of
-                  record.association(inverse_reflection.name).inversed_from(self)
-                end
+                association.set_inverse_instance(record)
               end
 
               saved = record.save(validate: !autosave)

activerecord/test/cases/associations/has_many_associations_test.rb

@@ -1440,7 +1440,7 @@ class HasManyAssociationsTest < ActiveRecord::TestCase
     assert_equal original_count, topic.reload.replies_count
   end
 
-  def test_calling_update_changing_ids_doesnt_change_counter_cache
+  def test_calling_update_changing_ids_changes_the_counter_cache
     topic1 = Topic.find(1)
     topic2 = Topic.find(3)
     original_count1 = topic1.replies.to_a.size
@@ -1458,6 +1458,24 @@ class HasManyAssociationsTest < ActiveRecord::TestCase
     assert_equal original_count2, topic2.reload.replies_count
   end
 
+  def test_calling_update_changing_ids_of_inversed_association_changes_the_counter_cache
+    assert_predicate Post.reflect_on_association(:comments), :has_inverse?
+
+    post1 = Post.first
+    post2 = Post.second
+
+    original_count1 = post1.comments.count
+    original_count2 = post2.comments.count
+
+    post1.comments.first.update(post_id: post2.id)
+    assert_equal original_count1 - 1, post1.reload.comments_count
+    assert_equal original_count2 + 1, post2.reload.comments_count
+
+    post2.comments.first.update(post_id: post1.id)
+    assert_equal original_count1, post1.reload.comments_count
+    assert_equal original_count2, post2.reload.comments_count
+  end
+
   def test_deleting_a_collection
     force_signal37_to_load_all_clients_of_firm
 

activerecord/test/cases/associations/inverse_associations_test.rb

@@ -664,6 +664,18 @@ class InverseHasManyTests < ActiveRecord::TestCase
     comment.body = "OMG"
     assert_equal comment.body, comment.children.first.parent.body
   end
+
+  def test_changing_the_association_id_makes_the_inversed_association_target_stale
+    post1 = Post.first
+    post2 = Post.second
+    comment = post1.comments.first
+
+    assert_same post1, comment.post
+
+    comment.update!(post_id: post2.id)
+
+    assert_equal post2, comment.post
+  end
 end
 
 class InverseBelongsToTests < ActiveRecord::TestCase