Eager loading won't mutate owner record

Since #31575, `BelongsToAssociation#target=` replaces owner record's
foreign key to fix an inverse association bug.

But the method is not only used for inverse association but also used
for eager loading/preloading, it caused some public behavior changes
(#32338, #32375).

To avoid any side-effect in loading associations, I reverted the
overriding `#target=`, then introduced `#inversed_from` to replace
foreign key in `set_inverse_instance`.

Closes #32375.

2 files changed, 19 insertions, 11 deletions

diff --git a/activerecord/lib/active_record/associations/association.rb b/activerecord/lib/active_record/associations/association.rb
index ca8c7794e0..d2934c830a 100644
--- a/activerecord/lib/active_record/associations/association.rb
+++ b/activerecord/lib/active_record/associations/association.rb
@@ -19,7 +19,6 @@ module ActiveRecord
     #         HasManyThroughAssociation + ThroughAssociation
     class Association #:nodoc:
       attr_reader :owner, :target, :reflection
-      attr_accessor :inversed
 
       delegate :options, to: :reflection
 
@@ -67,7 +66,7 @@ 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
+        !@inversed && loaded? && @stale_state != stale_state
       end
 
       # Sets the target of this association to <tt>\target</tt>, and the \loaded flag to +true+.
@@ -98,23 +97,24 @@ module ActiveRecord
 
       # Set the inverse association, if possible
       def set_inverse_instance(record)
-        if invertible_for?(record)
-          inverse = record.association(inverse_reflection_for(record).name)
-          inverse.target = owner
-          inverse.inversed = true
+        if inverse = inverse_association_for(record)
+          inverse.inversed_from(owner)
         end
         record
       end
 
       # Remove the inverse association, if possible
       def remove_inverse_instance(record)
-        if invertible_for?(record)
-          inverse = record.association(inverse_reflection_for(record).name)
-          inverse.target = nil
-          inverse.inversed = false
+        if inverse = inverse_association_for(record)
+          inverse.inversed_from(nil)
         end
       end
 
+      def inversed_from(record)
+        self.target = record
+        @inversed = !!record
+      end
+
       # Returns the class of the target. belongs_to polymorphic overrides this to look at the
       # polymorphic_type field on the owner.
       def klass
@@ -240,6 +240,12 @@ module ActiveRecord
           end
         end
 
+        def inverse_association_for(record)
+          if invertible_for?(record)
+            record.association(inverse_reflection_for(record).name)
+          end
+        end
+
         # Can be redefined by subclasses, notably polymorphic belongs_to
         # The record parameter is necessary to support polymorphic inverses as we must check for
         # the association in the specific class of the record.
diff --git a/activerecord/lib/active_record/associations/belongs_to_association.rb b/activerecord/lib/active_record/associations/belongs_to_association.rb
index c8716741b0..775559e224 100644
--- a/activerecord/lib/active_record/associations/belongs_to_association.rb
+++ b/activerecord/lib/active_record/associations/belongs_to_association.rb
@@ -16,7 +16,7 @@ module ActiveRecord
         end
       end
 
-      def target=(record)
+      def inversed_from(record)
         replace_keys(record)
         super
       end
@@ -53,6 +53,8 @@ module ActiveRecord
             decrement_counters
           end
 
+          replace_keys(record)
+
           self.target = record
         end