What determines the return value of `present?`?

Question

Some valid ActiveRecord objects return false for present?:

object.nil? # => false
object.valid? # => true
object.present? # => false
object.blank? # => true

I prefer object.present? over not object.nil?. What determines the return value of present?/blank??

EDIT: Found the answer: I had redefined the empty? method on this class, not the blank? or present? method; along the lines of @Simone's answer, blank? is using empty? behind the scenes.

http://stackoverflow.com/questions/885414/a-concise-explanation-of-nil-v-empty-v-blank-in-ruby-on-rails — Léo, Feb 15 '16 at 13:34
I prefer `object.present?` over `not object.nil?`--If you want something non-standard, then just define it by yourself and use it. If you don't want to do that, then don't prefer like that. — sawa, Feb 15 '16 at 13:47
*"Some valid `ActiveRecord` objects return `false` for `present?`"* - really? Could you give an example? — Stefan, Feb 15 '16 at 13:50

score 7 · Answer 1 · answered Feb 15 '16 at 13:38

present? is the opposite of blank?. blank? implementation depends on the type of object. Generally speaking, it returns true when the value is empty or like-empty.

You can get an idea looking at the tests for the method:

  BLANK = [ EmptyTrue.new, nil, false, '', '   ', "  \n\t  \r ", '　', "\u00a0", [], {} ]
  NOT   = [ EmptyFalse.new, Object.new, true, 0, 1, 'a', [nil], { nil => 0 } ]

  def test_blank
    BLANK.each { |v| assert_equal true, v.blank?,  "#{v.inspect} should be blank" }
    NOT.each   { |v| assert_equal false, v.blank?, "#{v.inspect} should not be blank" }
  end

  def test_present
    BLANK.each { |v| assert_equal false, v.present?, "#{v.inspect} should not be present" }
    NOT.each   { |v| assert_equal true, v.present?,  "#{v.inspect} should be present" }
  end

An object can define its own interpretation of blank?. For example

class Foo
  def initialize(value)
    @value = value
  end
  def blank?
    @value != "foo"
  end
end

Foo.new("bar").blank?
# => true

Foo.new("foo").blank?
# => false

If not specified, it will fallback to the closest implementation (e.g. Object).

Why `EmptyTrue` and `EmptyFalse` instead of the singleton TrueClass/FalseClass? — max, Feb 15 '16 at 13:49
@max take a look at the test, these are test classes with a custom implementation of `empty?`. And regarding `TrueClass`/`FalseClass` - try to create an instance via `TrueClass.new` (won't work). — Stefan, Feb 15 '16 at 13:52
I don't think you understood my question, why use a stand-in instead of `true` / `false`. It just seems weird to stub out the object under test. — max, Feb 15 '16 at 14:09

score 1 · Answer 2 · edited Feb 15 '16 at 14:09

1

From the documentation for Object#present?

An object is present if it's not blank.

In your case, object is blank, so present? will return false. Validity of the object does not really matter.

edited Feb 15 '16 at 14:09

Stefan

109,145
14
143
218

answered Feb 15 '16 at 13:36

eugen

8,916
11
57
65

So, the question is, why is my object blank? And I just figured it out: I redefined the `empty?` method and apparently it's using this. – Sasgorilla Feb 17 '16 at 00:37

What determines the return value of `present?`?

2 Answers2