class ActiveRecord::Validations::UniquenessValidator

def build_relation(klass, attribute, value)
  relation = klass.unscoped
  comparison = relation.bind_attribute(attribute, value) do |attr, bind|
    return relation.none! if bind.unboundable?
    if !options.key?(:case_sensitive) || bind.nil?
      klass.connection.default_uniqueness_comparison(attr, bind)
    elsif options[:case_sensitive]
      klass.connection.case_sensitive_comparison(attr, bind)
    else
      # will use SQL LOWER function before comparison, unless it detects a case insensitive collation
      klass.connection.case_insensitive_comparison(attr, bind)
    end
  end
  relation.where!(comparison)
end

def covered_by_unique_index?(klass, record, attribute, scope)
  @covered ||= self.attributes.map(&:to_s).select do |attr|
    attributes = scope + [attr]
    attributes = resolve_attributes(record, attributes)
    klass.connection.schema_cache.indexes(klass.table_name).any? do |index|
      index.unique &&
        index.where.nil? &&
        (Array(index.columns) - attributes).empty?
    end
  end
  @covered.include?(attribute.to_s)
end

def find_finder_class_for(record)
  class_hierarchy = [record.class]
  while class_hierarchy.first != @klass
    class_hierarchy.unshift(class_hierarchy.first.superclass)
  end
  class_hierarchy.detect { |klass| !klass.abstract_class? }
end

def initialize(options)
  if options[:conditions] && !options[:conditions].respond_to?(:call)
    raise ArgumentError, "#{options[:conditions]} was passed as :conditions but is not callable. " \
                         "Pass a callable instead: `conditions: -> { where(approved: true) }`"
  end
  unless Array(options[:scope]).all? { |scope| scope.respond_to?(:to_sym) }
    raise ArgumentError, "#{options[:scope]} is not supported format for :scope option. " \
      "Pass a symbol or an array of symbols instead: `scope: :user_id`"
  end
  super
  @klass = options[:class]
end

def map_enum_attribute(klass, attribute, value)
  mapping = klass.defined_enums[attribute.to_s]
  value = mapping[value] if value && mapping
  value
end

def resolve_attributes(record, attributes)
  attributes.flat_map do |attribute|
    reflection = record.class._reflect_on_association(attribute)
    if reflection.nil?
      attribute.to_s
    elsif reflection.polymorphic?
      [reflection.foreign_key, reflection.foreign_type]
    else
      reflection.foreign_key
    end
  end
end

def scope_relation(record, relation)
  Array(options[:scope]).each do |scope_item|
    scope_value = if record.class._reflect_on_association(scope_item)
      record.association(scope_item).reader
    else
      record.read_attribute(scope_item)
    end
    relation = relation.where(scope_item => scope_value)
  end
  relation
end

def validate_each(record, attribute, value)
  finder_class = find_finder_class_for(record)
  value = map_enum_attribute(finder_class, attribute, value)
  return if record.persisted? && !validation_needed?(finder_class, record, attribute)
  relation = build_relation(finder_class, attribute, value)
  if record.persisted?
    if finder_class.primary_key
      relation = relation.where.not(finder_class.primary_key => [record.id_in_database])
    else
      raise UnknownPrimaryKey.new(finder_class, "Cannot validate uniqueness for persisted record without primary key.")
    end
  end
  relation = scope_relation(record, relation)
  if options[:conditions]
    conditions = options[:conditions]
    relation = if conditions.arity.zero?
      relation.instance_exec(&conditions)
    else
      relation.instance_exec(record, &conditions)
    end
  end
  if relation.exists?
    error_options = options.except(:case_sensitive, :scope, :conditions)
    error_options[:value] = value
    record.errors.add(attribute, :taken, **error_options)
  end
end

def validation_needed?(klass, record, attribute)
  return true if options[:conditions] || options.key?(:case_sensitive)
  scope = Array(options[:scope])
  attributes = scope + [attribute]
  attributes = resolve_attributes(record, attributes)
  return true if attributes.any? { |attr| record.attribute_changed?(attr) ||
                                          record.read_attribute(attr).nil? }
  !covered_by_unique_index?(klass, record, attribute, scope)
end