brand.rb

class Brand < ActiveRecord::Base
  validates_with BrandValidator

  # ...
end

brand_validator.rb

class BrandValidator < ActiveModel::Validator
  def validate(record)
    must_not_be_white_and_blacklisted(record)
    must_not_be_synonym_of_self(record)
  end

  private

  def must_not_be_white_and_blacklisted(record)
    if record.whitelisted && record.blacklisted
      record.errors[:status] << 'must not be white- and blacklisted'
    end
  end

  def must_not_be_synonym_of_self(record)
    if record.synonym == record
      record.errors[:synonym] << 'is itself'
    end
  end
end

brand_validator_spec.rb

require 'active_model'
require 'support/validations'
require_relative '../../app/validators/brand_validator'

class Validatable
  include ActiveModel::Validations
  validates_with BrandValidator
end

VALID_ATTRIBUTES = {
  :whitelisted => false,
  :blacklisted => false,
  :synonym => nil,
}

describe BrandValidator do
  subject { Validatable.new }
  before { subject.stub(VALID_ATTRIBUTES) }

  context 'in normal operation' do
    it { should be_valid }
  end

  context 'with a brand that is both white- and blacklisted' do
    before { subject.stub(:whitelisted => true, :blacklisted => true) }
    it { should have(1).error_on(:status) }
  end

  context 'with a brand that is a synonym of itself' do
    before { subject.stub(:synonym => subject) }
    it { should have(1).error_on(:synonym) }
  end
end