gistfile1.txt

[101] pry(main)> ParanoidAttributes.only(Person.all, :id, :first_name).first.first_name
=> "Harry"

[102] pry(main)> ParanoidAttributes.only(Person.all, :id, :first_name).first.last_name
ArgumentError: Accessing excluded field last_name

[103] pry(main)> ParanoidAttributes.only(Person.all, :id, :first_name).first.bla
NoMethodError: undefined method `bla' for #<Person:0x007fe9ebbb2768>

gistfile2.rb

class ParanoidAttributes

  include Enumerable

  attr_accessor :criteria
  attr_accessor :klass
  attr_accessor :blacklisted_attributes
  attr_accessor :records

  def self.only(criteria, *attrs)
    self.new(criteria, :whitelist, attrs)
  end

  def self.without(criteria, *attrs)
    self.new(criteria, :blacklist, attrs)
  end

  def initialize(criteria, action, attrs)
    self.criteria = criteria
    self.klass = criteria.klass

    if action == :whitelist
      whitelist(attrs)
    elsif action == :blacklist
      blacklist(attrs)
    end

    self.protect_attributes
  end

  def whitelist(attrs)
    self.records = criteria.only(*attrs)
    self.blacklisted_attributes = klass.fields.keys - attrs.map(&:to_s)
  end

  def blacklist(attrs)
    self.records = criteria.without(*attrs)
    self.blacklisted_attributes = attrs.map(&:to_s)
  end

  def protect_attributes
    blacklisted_attributes = self.blacklisted_attributes

    klass.class_eval do
      blacklisted_attributes.each do |attr|
        define_method attr do
          raise ArgumentError, "Accessing excluded field #{attr}"
        end
      end
    end
  end

  def each &block
    records.each do |record|
      if block_given?
        block.call record
      else
        yield record
      end
    end
  end

end