Improve support for non Active Record objects on `validates_associated`

Skipping `marked_for_destruction?` when the associated object does not responds
to it make easier to validate virtual associations built on top of Active Model
objects and/or serialized objects that implement a `valid?` instance method.

activerecord/CHANGELOG.md

@@ -1,3 +1,11 @@
+*   Improve support for non Active Record objects on `validates_associated`
+
+    Skipping `marked_for_destruction?` when the associated object does not responds
+    to it make easier to validate virtual associations built on top of Active Model
+    objects and/or serialized objects that implement a `valid?` instance method.
+
+    *Kassio Borges*, *Lucas Mazza*
+
 *   Change connection management middleware to return a new response with
     a body proxy, rather than mutating the original.
 

activerecord/lib/active_record/validations/associated.rb

@@ -2,10 +2,16 @@ module ActiveRecord
   module Validations
     class AssociatedValidator < ActiveModel::EachValidator #:nodoc:
       def validate_each(record, attribute, value)
-        if Array.wrap(value).reject {|r| r.marked_for_destruction? || r.valid?}.any?
-          record.errors.add(attribute, :invalid, options.merge(:value => value))
+        if Array(value).reject { |r| valid_object?(r) }.any?
+          record.errors.add(attribute, :invalid, options.merge(value: value))
         end
       end
+
+      private
+
+        def valid_object?(record)
+          (record.respond_to?(:marked_for_destruction?) && record.marked_for_destruction?) || record.valid?
+        end
     end
 
     module ClassMethods

activerecord/test/cases/validations/association_validation_test.rb

@@ -45,6 +45,18 @@ class AssociationValidationTest < ActiveRecord::TestCase
     assert t.valid?
   end
 
+  def test_validates_associated_without_marked_for_destruction
+    reply = Class.new do
+      def valid?
+        true
+      end
+    end
+    Topic.validates_associated(:replies)
+    t = Topic.new
+    t.define_singleton_method(:replies) { [reply.new] }
+    assert t.valid?
+  end
+
   def test_validates_associated_with_custom_message_using_quotes
     Reply.validates_associated :topic, :message=> "This string contains 'single' and \"double\" quotes"
     Topic.validates_presence_of :content