Validates that the specified attribute is present.

For most property types "being present" is the same as being "not blank" as determined by the attribute‘s blank? method. However, in the case of Boolean, "being present" means not nil; i.e. true or false.

@note

  dm-core's support lib adds the blank? method to many classes,

@see lib/dm-core/support/blank.rb (dm-core) for more information.

@example Usage

  require 'dm-validations'

  class Page
    include DataMapper::Resource

    property :required_attribute, String
    property :another_required, String
    property :yet_again, String

    validates_presence_of :required_attribute
    validates_presence_of :another_required, :yet_again

    # a call to valid? will return false unless
    # all three attributes are !blank?
  end