In Rails 3.2.13 using accepts_nested_attributes_for, a scoped validates_uniqueness_of validator will incorrectly reject unique values.

Rails uniqueness validation bug

# In Rails 3.2.13 using accepts_nested_attributes_for, a scoped 
# validates_uniqueness_of validator will incorrectly reject unique values.

# Below are two models, Question and AnswerChoice. The validates_uniqueness_of
# validator in AnswerChoice says that AnswerChoice label must be unique within a
# Question. This validator incorrectly rejects unique labels when they are 
# reordered. Follow these steps to see the bug.
#
# This occurs when using nested resources and allowing users to edit the 
# Question and AnswerChoices in a single form. The form looks like this:
#
# Question: _______________________________
#     Answer 1: ___________________________
# 	  Answer 2: ___________________________
# 	  Answer 3: ___________________________
#
# 1. Create a new question.
# 2. Add three answer choices setting the label of the first choice to 
#    "One", the label of the second choice to "Two" and the label of the 
#    third choice to "Three".
# 3. Save the Question and all the answer choices.
# 4. Edit the question again, change the labels to "Two", "Three", "One", 
#    in that order.
# 5. Save the Question.
#
# Expected: No validation error.
# Actual: You get an error saying that the answer choices must be unique.


# Schema info for Question
# Table name: questions
#
#  id                     :integer          not null, primary key
#  prompt                 :string(500)      not null
#  sort_order             :integer          not null
#  created_by             :integer          default(0), not null
#  updated_by             :integer
#  created_at             :datetime         not null
#  updated_at             :datetime         not null

class Question < ActiveRecord::Base

  has_many :answer_choices
  accepts_nested_attributes_for :choices, :allow_destroy => true

  validates_presence_of :prompt
  validates :choices, length: { minimum: 2, message: "Question must have at least 2 choices." }

  attr_accessible :prompt, :answer_choices_attributes

end

# Schema for AnswerChoice
# id                    :integer             not null primary key
# question_id           :integer             not null
# value                 :integer             
# label                 :string              
# sort_order            :integer             not null
# is_correct            :boolean             not null default false
# created_by            :integer             not null default 0
# updated_by            :integer             
# created_at            :datetime            not null
# updated_at            :datetime            not null

class AnswerChoice < ActiveRecord::Base

  belongs_to :question

  attr_accessible :value, :label, :sort_order, :is_correct

  validates :sort_order, presence: true
  validates_length_of :label, :maximum => 400

  validates_uniqueness_of :label, :scope => [:question_id], :allow_blank => false, :case_sensitive => false, message: "Answer choice must be unique."

end


# The following validator works without error for this case:

class UniqueChoiceValidator < ActiveModel::Validator
  def validate(record)
    exists = {}
    record.choices.each_with_index do |choice, index|
      choice.label ||= ''  # conver nils so downcase doesn't raise
      if exists[choice.label.downcase]
        record.errors[:choices] << "Choices must be unique"
        record.choices[index].errors[:label] << "Choices must be unique!"
      end
      exists[choice.label.downcase] = true
    end
  end
end