Rails existence confirmation method properly used memo (any? / Empty? / Present? (! Blank?) / Nil?)


In the Rails view, there was a scene to write a process like "display if there is an image". There are several methods for checking the existence, including those for Ruby and those for Rails. After a little research, I selected the method "OK, let's go with this!" And submitted it for review, but it was completely crushed, so I will keep it as a memorandum so that I will not make a mistake in the future.


macOS Catalina Version 10.15.5 Ruby: 2.6.5 Ruby on Rails: 6.0.3

Survey target


There have been similar articles in the past, and most of them are actually "ruby's authenticity judgment method (nil? / Empty? / Blank? / Present?) Turned out to be interesting //qiita.com/go_d_eye_0505/items/541110cb9821734b0623) ”. On top of that, the reasons for posting this time are as follows.

--The any? Method was not mentioned in the above post, but I also wanted to confirm the behavior of any? (Because I faced the problem of array entanglement this time). --In the summary table of the above post,
-"** A method to ask if there is ** (present? Etc., returns true if there is something)"
-"** Not ** or not" Method to listen to (empty? Etc., returns true if there is nothing) "
was written side by side, but a table with the direction of the condition aligned, such as" returns 〇〇 if there is something " I wanted (a table where present? And not empty? Can be compared)
* Of course, the merit of the above table is that it is easy to understand that present? And blank? Are in a back relationship.

In addition, blank? Is excluded (because the definition of present? Is! Blank?).

Survey method

Use rails console.

test =XXX ← Substitute variously here


Survey results

any? !empty?
(empty?Denial of)
(nil?Denial of)
Ruby/Rails Ruby Ruby Rails Ruby
NoMethodError NoMethodError true true
NoMethodError true true true
{key: value}
true true true true
true true true true
true NoMethodError NoMethodError true true
false NoMethodError NoMethodError false true
nil NoMethodError NoMethodError false false
NoMethodError false false true
" "
Half-width space
NoMethodError TRUE FALSE true
Empty hash
false false false true
Empty array
false false false true
Array(The element is nil)

The point

From this result, I have emphasized the points that I should be careful about in the future in capital letters in the table, but I will describe them again below.

  1. Half-width space is regarded as ~~ existing ~~ ** not empty ** in ! Empty? and returns true, and in present? it is regarded as empty and returns false
    ↑ (Correction) We received comments from @scivola, and corrected and added. Thank you very much.
    (Addition) Note that present? (Motoi blank?) Has a wide range of empty judgments.
    Example: In addition to false, nil, and half-width spaces, full-width spaces, tabs (\ t), line breaks (\ n), Unicode (\ unnnn), etc. are also judged to be empty.
  2. Applying ʻany?` to an array returns true if there are ** true elements (= non-nil elements) **. Other methods return true if there is an element, whether true or not.
  3. If you ask the plain nil to! Nil? , It returns false, but if you ask the array made of[nil], it returns true.


In Rails, there is also a method called ʻexists?. This seems to be used to check if the database has data with specific conditions, and it was a method defined in ʻActiveRecord :: Base. I didn't know about it and initially included it in the investigation, but all returned NoMethodError.


This is Qiita's first post. I think the content is simple and natural, but it was a good opportunity to deepen understanding through the process of examining and organizing for output. I would like to regularly post articles that will be useful to me (≒ others) in the future.

Reference URL

As a result of verifying the authenticity judgment method of ruby (nil? / Empty? / Blank? / Present?), I found it interesting

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