Don't validate non dirty association targets

Fixes #36581.

This fixes an issue where validations would return differently when a previously saved invalid association was loaded between calls:

    assert_equal true, squeak.valid?
    assert_equal true, squeak.mouse.present?
    assert_equal true, squeak.valid?

Here the second assert would return

    Expected: true
    Actual: false

Limiting validations to associations that would be normally saved (using autosave: true) due to changes means that loading invalid associated relations will not change the return value of the parent relations's `valid?` method.

activerecord/lib/active_record/autosave_association.rb

@@ -302,7 +302,7 @@ module ActiveRecord
       def validate_single_association(reflection)
         association = association_instance_get(reflection.name)
         record      = association && association.reader
-        association_valid?(reflection, record) if record
+        association_valid?(reflection, record) if record && record.changed_for_autosave?
       end
 
       # Validate the associated records if <tt>:validate</tt> or

activerecord/test/cases/autosave_association_test.rb

@@ -13,12 +13,14 @@ require "models/developer"
 require "models/computer"
 require "models/invoice"
 require "models/line_item"
+require "models/mouse"
 require "models/order"
 require "models/parrot"
 require "models/pirate"
 require "models/project"
 require "models/ship"
 require "models/ship_part"
+require "models/squeak"
 require "models/tag"
 require "models/tagging"
 require "models/treasure"
@@ -386,6 +388,20 @@ class TestDefaultAutosaveAssociationOnABelongsToAssociation < ActiveRecord::Test
 
     assert_predicate auditlog, :valid?
   end
+
+  def test_validation_does_not_validate_non_dirty_association_target
+    mouse = Mouse.create!(name: "Will")
+    Squeak.create!(mouse: mouse)
+
+    mouse.name = nil
+    mouse.save! validate: false
+
+    squeak = Squeak.last
+
+    assert_equal true, squeak.valid?
+    assert_equal true, squeak.mouse.present?
+    assert_equal true, squeak.valid?
+  end
 end
 
 class TestDefaultAutosaveAssociationOnAHasManyAssociationWithAcceptsNestedAttributes < ActiveRecord::TestCase

activerecord/test/models/mouse.rb

@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+
+class Mouse < ActiveRecord::Base
+  has_many :squeaks, autosave: true
+  validates :name, presence: true
+end

activerecord/test/models/squeak.rb

@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+
+class Squeak < ActiveRecord::Base
+  belongs_to :mouse
+  accepts_nested_attributes_for :mouse
+end

activerecord/test/schema/schema.rb

@@ -563,6 +563,10 @@ ActiveRecord::Schema.define do
     t.string   :type
   end
 
+  create_table :mice, force: true do |t|
+    t.string   :name
+  end
+
   create_table :movies, force: true, id: false do |t|
     t.primary_key :movieid
     t.string      :name
@@ -843,6 +847,10 @@ ActiveRecord::Schema.define do
     end
   end
 
+  create_table :squeaks, force: true do |t|
+    t.integer :mouse_id
+  end
+
   create_table :prisoners, force: true do |t|
     t.belongs_to :ship
   end