`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]
