invalid?

Returns true if the specified attribute has errors associated with it.

  class Company < ActiveRecord::Base
    validates_presence_of :name, :address, :email
    validates_length_of :name, :in => 5..30
  end

  company = Company.create(:address => '123 First St.')
  company.errors.invalid?(:name)      # => true
  company.errors.invalid?(:address)   # => false