validates_uniqueness_of when updating car.number

When updating car info, the validation process fails because I have validates_uniqueness_of :number

class Car < ActiveRecord::Base
validates :number,numericality: true, length: {is: 7 }
validates :number, :name, presence:true
validates_uniqueness_of :number, :message => "מספר רכב זה קיים במערבת"
belongs_to :owner
has_many :visits
end

I need validation to pass, if the original value was not changed, validation on_create would not help since I still need validation when updating.

Any help would be really appreciated.