1 files changed, 9 insertions, 3 deletions
diff --git a/activerecord/lib/active_record/autosave_association.rb b/activerecord/lib/active_record/autosave_association.rb
index d35bc3e794..fc12c3f45a 100644
--- a/activerecord/lib/active_record/autosave_association.rb
+++ b/activerecord/lib/active_record/autosave_association.rb
@@ -140,6 +140,8 @@ module ActiveRecord
 
     included do
       Associations::Builder::Association.extensions << AssociationBuilderExtension
+      mattr_accessor :index_nested_attribute_errors, instance_writer: false
+      self.index_nested_attribute_errors = false
     end
 
     module ClassMethods # :nodoc:
@@ -315,7 +317,7 @@ module ActiveRecord
       def validate_collection_association(reflection)
         if association = association_instance_get(reflection.name)
           if records = associated_records_to_validate_or_save(association, new_record?, reflection.options[:autosave])
-            records.each { |record| association_valid?(reflection, record) }
+            records.each_with_index { |record, index| association_valid?(reflection, record, index) }
           end
         end
       end
@@ -323,14 +325,18 @@ module ActiveRecord
       # Returns whether or not the association is valid and applies any errors to
       # 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)
+      def association_valid?(reflection, record, index=nil)
         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)
           if reflection.options[:autosave]
             record.errors.each do |attribute, message|
-              attribute = "#{reflection.name}.#{attribute}"
+              if index.nil? || (!reflection.options[:index_errors] && !ActiveRecord::Base.index_nested_attribute_errors)
+                attribute = "#{reflection.name}.#{attribute}"
+              else
+                attribute = "#{reflection.name}[#{index}].#{attribute}"
+              end
               errors[attribute] << message
               errors[attribute].uniq!
             end