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]

1 files changed, 4 insertions, 0 deletions

diff --git a/guides/source/configuring.md b/guides/source/configuring.md
index aa66376d5d..4e06c8000f 100644
--- a/guides/source/configuring.md
+++ b/guides/source/configuring.md
@@ -314,6 +314,10 @@ All these configuration options are delegated to the `I18n` library.
   by a query exceeds the threshold, a warning is logged. This can be used to
   identify queries which might be causing memory bloat.
 
+* `config.active_record.index_nested_attribute_errors` allows errors for nested
+  has_many relationships to be displayed with an index as well as the error.
+  Defaults to false.
+
 The MySQL adapter adds one additional configuration option:
 
 * `ActiveRecord::ConnectionAdapters::MysqlAdapter.emulate_booleans` controls whether Active Record will consider all `tinyint(1)` columns in a MySQL database to be booleans and is true by default.