Always perform validations on nested attribute associations

Collection associations would have already been validated, but singular
associations were not.

Fixes #18735.

activerecord/lib/active_record/autosave_association.rb

@@ -177,10 +177,8 @@ module ActiveRecord
         # before actually defining them.
         def add_autosave_association_callbacks(reflection)
           save_method = :"autosave_associated_records_for_#{reflection.name}"
-          validation_method = :"validate_associated_records_for_#{reflection.name}"
-          collection = reflection.collection?
 
-          if collection
+          if reflection.collection?
             before_save :before_save_collection_association
 
             define_non_cyclic_method(save_method) { save_collection_association(reflection) }
@@ -204,8 +202,18 @@ module ActiveRecord
             before_save save_method
           end
 
+          define_autosave_validation_callbacks(reflection)
+        end
+
+        def define_autosave_validation_callbacks(reflection)
+          validation_method = :"validate_associated_records_for_#{reflection.name}"
           if reflection.validate? && !method_defined?(validation_method)
-            method = (collection ? :validate_collection_association : :validate_single_association)
+            if reflection.collection?
+              method = :validate_collection_association
+            else
+              method = :validate_single_association
+            end
+
             define_non_cyclic_method(validation_method) do
               send(method, reflection)
               # TODO: remove the following line as soon as the return value of

activerecord/lib/active_record/nested_attributes.rb

@@ -312,6 +312,7 @@ module ActiveRecord
         attr_names.each do |association_name|
           if reflection = _reflect_on_association(association_name)
             reflection.autosave = true
+            define_autosave_validation_callbacks(reflection)
 
             nested_attributes_options = self.nested_attributes_options.dup
             nested_attributes_options[association_name.to_sym] = options

activerecord/test/cases/nested_attributes_test.rb

@@ -1047,4 +1047,11 @@ class TestHasManyAutosaveAssociationWhichItselfHasAutosaveAssociations < ActiveR
       ship.save!
     end
   end
+
+  test "nested singular associations are validated" do
+    part = ShipPart.new(name: "Stern", ship_attributes: { name: nil })
+
+    assert_not part.valid?
+    assert_equal ["Ship name can't be blank"], part.errors.full_messages
+  end
 end

activerecord/test/models/ship_part.rb

@@ -2,6 +2,7 @@ class ShipPart < ActiveRecord::Base
   belongs_to :ship
   has_many :trinkets, :class_name => "Treasure", :as => :looter
   accepts_nested_attributes_for :trinkets, :allow_destroy => true
+  accepts_nested_attributes_for :ship
 
   validates_presence_of :name
-end
+end