model - Rails: How do I run a before_save only if certain conditions are met? -

i have before_save method call renames uploaded image.

before_save :randomize_file_name  def randomize_file_name   extension = file.extname(screen_file_name).downcase   key = activesupport::securerandom.hex(8)   self.screen.instance_write(:file_name, "#{key}#{extension}") end 

that method part of item model.

that works great when create new item or need update image associated item...but problem if need update item not image, randomize_file_name method still gets run , renames file in database (though not file itself, obviously).

so, i'm thinking need figure out way run randomize_file_name if file included in form submission...but i'm not sure how pull off.

use dirty objects.

before_save :randomize_file_name  def randomize_file_name   # assuming field holds name   # called screen_file_name   if screen_file_name_changed?     extension = file.extname(screen_file_name).downcase     key = activesupport::securerandom.hex(8)     self.screen.instance_write(:file_name, "#{key}#{extension}")   end end