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]

7 files changed, 91 insertions, 4 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 904ac5c26a..5dbd0f6332 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,31 @@
+*   Add option to index errors in nested attributes
+
+    For models which have nested attributes, errors within those models will
+    now be indexed if :index_errors is specified when defining a
+    has_many relationship, or if its set in the global config.
+
+    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 and Terence Sun*
+
 *   Fixed a bug where uniqueness validations would error on out of range values,
     even if an validation should have prevented it from hitting the database.
 
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
diff --git a/activerecord/test/cases/autosave_association_test.rb b/activerecord/test/cases/autosave_association_test.rb
index 8f0d7bd037..c089deefa7 100644
--- a/activerecord/test/cases/autosave_association_test.rb
+++ b/activerecord/test/cases/autosave_association_test.rb
@@ -24,6 +24,8 @@ require 'models/molecule'
 require 'models/member'
 require 'models/member_detail'
 require 'models/organization'
+require 'models/guitar'
+require 'models/tuning_peg'
 
 class TestAutosaveAssociationsInGeneral < ActiveRecord::TestCase
   def test_autosave_validation
@@ -385,6 +387,40 @@ class TestDefaultAutosaveAssociationOnAHasManyAssociationWithAcceptsNestedAttrib
     assert_not molecule.persisted?, 'Molecule should not be persisted when its electrons are invalid'
   end
 
+  def test_errors_should_be_indexed_when_passed_as_array
+    guitar = Guitar.new
+    tuning_peg_valid = TuningPeg.new
+    tuning_peg_valid.pitch = 440.0
+    tuning_peg_invalid = TuningPeg.new
+
+    guitar.tuning_pegs = [tuning_peg_valid, tuning_peg_invalid]
+
+    assert_not tuning_peg_invalid.valid?
+    assert tuning_peg_valid.valid?
+    assert_not guitar.valid?
+    assert_equal ["is not a number"], guitar.errors["tuning_pegs[1].pitch"]
+    assert_not_equal ["is not a number"], guitar.errors["tuning_pegs.pitch"]
+  end
+
+  def test_errors_should_be_indexed_when_global_flag_is_set
+    old_attribute_config = ActiveRecord::Base.index_nested_attribute_errors
+    ActiveRecord::Base.index_nested_attribute_errors = true
+
+    molecule = Molecule.new
+    valid_electron = Electron.new(name: 'electron')
+    invalid_electron = Electron.new
+
+    molecule.electrons = [valid_electron, invalid_electron]
+
+    assert_not invalid_electron.valid?
+    assert valid_electron.valid?
+    assert_not molecule.valid?
+    assert_equal ["can't be blank"], molecule.errors["electrons[1].name"]
+    assert_not_equal ["can't be blank"], molecule.errors["electrons.name"]
+  ensure
+    ActiveRecord::Base.index_nested_attribute_errors = old_attribute_config
+  end
+
   def test_valid_adding_with_nested_attributes
     molecule = Molecule.new
     valid_electron = Electron.new(name: 'electron')
diff --git a/activerecord/test/models/guitar.rb b/activerecord/test/models/guitar.rb
new file mode 100644
index 0000000000..cd068ff53d
--- /dev/null
+++ b/activerecord/test/models/guitar.rb
@@ -0,0 +1,4 @@
+class Guitar < ActiveRecord::Base
+  has_many :tuning_pegs, index_errors: true
+  accepts_nested_attributes_for :tuning_pegs
+end
diff --git a/activerecord/test/models/tuning_peg.rb b/activerecord/test/models/tuning_peg.rb
new file mode 100644
index 0000000000..1252d6dc1d
--- /dev/null
+++ b/activerecord/test/models/tuning_peg.rb
@@ -0,0 +1,4 @@
+class TuningPeg < ActiveRecord::Base
+  belongs_to :guitar
+  validates_numericality_of :pitch
+end
diff --git a/activerecord/test/schema/schema.rb b/activerecord/test/schema/schema.rb
index 7b42f8a4a5..3d39f922f3 100644
--- a/activerecord/test/schema/schema.rb
+++ b/activerecord/test/schema/schema.rb
@@ -336,6 +336,10 @@ ActiveRecord::Schema.define do
     t.column :key, :string
   end
 
+  create_table :guitar, force: true do |t|
+    t.string :color
+  end
+
   create_table :inept_wizards, force: true do |t|
     t.column :name, :string, null: false
     t.column :city, :string, null: false
@@ -789,6 +793,11 @@ ActiveRecord::Schema.define do
     t.belongs_to :ship
   end
 
+  create_table :tuning_pegs, force: true do |t|
+    t.integer :guitar_id
+    t.float :pitch
+  end
+
   create_table :tyres, force: true do |t|
     t.integer :car_id
   end