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.

1 files changed, 16 insertions, 0 deletions

diff --git a/activerecord/test/cases/autosave_association_test.rb b/activerecord/test/cases/autosave_association_test.rb
index 2d223a3035..3528ac045f 100644
--- a/activerecord/test/cases/autosave_association_test.rb
+++ b/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