Correctly ignore `mark_for_destruction` without `autosave`

As per the docs, `mark_for_destruction` should do nothing if `autosave`
is not set to true. We normally persist associations on a record no
matter what if the record is a new record, but we were always skipping
records which were `marked_for_destruction?`.

Fixes #20882

activerecord/CHANGELOG.md

+*   Correctly ignore `mark_for_destruction` when `autosave` isn't set to `true`
+    when validating associations.
+
+    Fixes #20882.
+
+    *Sean Griffin*
+
 *   Fix a bug where counter_cache doesn't always work with  polymorphic
     relations.
 

activerecord/lib/active_record/autosave_association.rb

@@ -325,7 +325,7 @@ def validate_collection_association(reflection)
       # the parent, <tt>self</tt>, if it wasn't. Skips any <tt>:autosave</tt>
       # enabled records if they're marked_for_destruction? or destroyed.
       def association_valid?(reflection, record)
-        return true if record.destroyed? || record.marked_for_destruction?
+        return true if record.destroyed? || (reflection.options[:autosave] && record.marked_for_destruction?)
 
         validation_context = self.validation_context unless [:create, :update].include?(self.validation_context)
         unless valid = record.valid?(validation_context)

activerecord/test/cases/autosave_association_test.rb

@@ -1154,6 +1154,13 @@ def save(*args)
   def test_should_not_load_the_associated_model
     assert_queries(1) { @pirate.catchphrase = 'Arr'; @pirate.save! }
   end
+
+  def test_mark_for_destruction_is_ignored_without_autosave_true
+    ship = ShipWithoutNestedAttributes.new(name: "The Black Flag")
+    ship.parts.build.mark_for_destruction
+
+    assert_not ship.valid?
+  end
 end
 
 class TestAutosaveAssociationOnAHasOneThroughAssociation < ActiveRecord::TestCase

activerecord/test/models/ship.rb

@@ -22,6 +22,7 @@ def cancel_save_callback_method
 class ShipWithoutNestedAttributes < ActiveRecord::Base
   self.table_name = "ships"
   has_many :prisoners, inverse_of: :ship, foreign_key: :ship_id
+  has_many :parts, class_name: "ShipPart", foreign_key: :ship_id
 
   validates :name, presence: true
 end