Add add_error_to option

[flyinggrizzly]

In cases where case/accent insensitive collations are not available, it
is useful to add a <attr>_normalized proxy column to drive unique
indexes.

In these cases, should the uniqueness constraint be violated, we want to
show the user errors referencing their actual input, without the
_normalized field name, nor the normalized value.

Usage

class MyModel < ApplicationRecord
  rescue_from_duplicate :name_normalized, add_error_to: :name
end

elmo = MyModel.create!(name: "Elmo")
elmo.name
#=> "Elmo"
elmo.name_normalized
#=> "elmo"

eelmo = MyModel.build(name: "Élmo")
eelmo.valid?
#=> false
eelmo.errors[:name]
#=> "is not unique"

[flyinggrizzly]

Alternative pattern

Could also be add_error_to <attr>, via: <attr>_normalized, which would put all behavior on <attr>_normalized, but redirect all errors to <attr>.

Means we don't have doubled validations and error rescuing.

[flyinggrizzly]

Handling with a simple override of #create_or_update instead.