validate :uniqueness_of_name

def uniqueness_of_name
   existing_record = Model.find_by_name(name)
   unless existing_record.nil?
     errors.add(:name, "Record #{existing_record.id} already has the name #{name}")
   end
end

class NameValidator
  def validate record
    unless record.name.include? 'somestring'
      record.errors[:name] << "#{record.name} is not valid"
    end
  end
end

validate :uniqueness_of_name

def uniqueness_of_name(current_record)
    existing_record = Model.find_by_name(name)
    unless existing_record.nil? || existing_record.id == current_record.id
        errors.add(:name, "Record #{existing_record.id} already has the name #{name}")
    end
end