validates_absence_of

Validates that the specified attributes are blank (as defined by Object#blank?). Happens by default on save.

class Person < ActiveRecord::Base
  validates_absence_of :first_name
end

The first_name attribute must be in the object and it must be blank.

Configuration options:

• :message - A custom error message (default is: “must be blank”).

There is also a list of default options supported by every validator: :if, :unless, :on, :allow_nil, :allow_blank, and :strict. See ActiveModel::Validation#validates for more information