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.

2 files changed, 12 insertions, 0 deletions

diff --git a/activerecord/test/models/mouse.rb b/activerecord/test/models/mouse.rb
new file mode 100644
index 0000000000..75a55c125d
--- /dev/null
+++ b/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
diff --git a/activerecord/test/models/squeak.rb b/activerecord/test/models/squeak.rb
new file mode 100644
index 0000000000..e0a643c238
--- /dev/null
+++ b/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