ruby on rails - Call before_update if an attribute is filled -

what way call before_update trigger if particular attribute filled?

here part of code:

before_update { self.gravatar = gravatar.downcase }  validates :gravatar, :on => :update, length: { maximum: 128 },                     format: { with: valid_gravatar_regex },                      allow_blank: true 

indeed want call downcase function on attribute if 1 filled in edit form.

i have following error:

undefined method `downcase' nil:nilclass

which logical.

like ? before_update { self.gravatar = gravatar.downcase if gravatar.present? }