4 files changed, 16 insertions, 1 deletions
diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 3fa24f3837..aa2438fb3f 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,10 @@
+*   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.
 
diff --git a/activerecord/lib/active_record/autosave_association.rb b/activerecord/lib/active_record/autosave_association.rb
index 5f38bd51f6..dbb0e2fab2 100644
--- a/activerecord/lib/active_record/autosave_association.rb
+++ b/activerecord/lib/active_record/autosave_association.rb
@@ -325,7 +325,7 @@ module ActiveRecord
       # 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)
diff --git a/activerecord/test/cases/autosave_association_test.rb b/activerecord/test/cases/autosave_association_test.rb
index 6b69b96e21..37ec3f1106 100644
--- a/activerecord/test/cases/autosave_association_test.rb
+++ b/activerecord/test/cases/autosave_association_test.rb
@@ -1154,6 +1154,13 @@ class TestAutosaveAssociationOnAHasOneAssociation < ActiveRecord::TestCase
   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
diff --git a/activerecord/test/models/ship.rb b/activerecord/test/models/ship.rb
index fcc533380b..95172e4d3e 100644
--- a/activerecord/test/models/ship.rb
+++ b/activerecord/test/models/ship.rb
@@ -22,6 +22,7 @@ end
 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