Errors can be indexed with nested attributes

`has_many` can now take `index_errors: true` as an
option.  When this is enabled, errors for nested models will be
returned alongside an index, as opposed to just the nested model name.
This option can also be enabled (or disabled) globally through
`ActiveRecord::Base.index_nested_attribute_errors`

E.X.

```ruby
class Guitar < ActiveRecord::Base
  has_many :tuning_pegs
  accepts_nested_attributes_for :tuning_pegs
end

class TuningPeg < ActiveRecord::Base
  belongs_to :guitar
  validates_numericality_of :pitch
end
```

 - Old style
 - `guitar.errors["tuning_pegs.pitch"] = ["is not a number"]`

 - New style (if defined globally, or set in has_many_relationship)
 - `guitar.errors["tuning_pegs[1].pitch"] = ["is not a number"]`

[Michael Probber, Terence Sun]

2 files changed, 10 insertions, 4 deletions

diff --git a/activerecord/lib/active_record/associations/builder/has_many.rb b/activerecord/lib/active_record/associations/builder/has_many.rb
index 1c1b47bd56..eda3b74528 100644
--- a/activerecord/lib/active_record/associations/builder/has_many.rb
+++ b/activerecord/lib/active_record/associations/builder/has_many.rb
@@ -5,7 +5,7 @@ module ActiveRecord::Associations::Builder
     end
 
     def self.valid_options(options)
-      super + [:primary_key, :dependent, :as, :through, :source, :source_type, :inverse_of, :counter_cache, :join_table, :foreign_type]
+      super + [:primary_key, :dependent, :as, :through, :source, :source_type, :inverse_of, :counter_cache, :join_table, :foreign_type, :index_errors]
     end
 
     def self.valid_dependent_options
diff --git a/activerecord/lib/active_record/autosave_association.rb b/activerecord/lib/active_record/autosave_association.rb
index 0792d19c3e..3c0a39d10f 100644
--- a/activerecord/lib/active_record/autosave_association.rb
+++ b/activerecord/lib/active_record/autosave_association.rb
@@ -141,6 +141,8 @@ 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
@@ -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? || 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