How to let `self` work in a class

Question

I want to downcase a user's email address before saving it to database in rails. I am at this page using this code:

class User < ActiveRecord::Base
  before_save {self.email = email.downcase}
end

It also works if I write self.email = self.email.downcase, but it doesn't work if I write email = email.downcase. Under which condition can self be omitted?

in the latter one,any error you found..? – Arup Rakshit Nov 13 '13 at 05:34 — Arup Rakshit, Nov 13 '13 at 05:34

score 7 · Accepted Answer · edited May 23 '17 at 10:24

7

self must always be supplied when wishing to use a setter¹ on the left-hand side of an assignment, e.g. self.x = val or self.x += val².

self can be omitted for getters (which are just normal method calls) when there is not a local variable that shadows the method.

Expressions of the form x = val or x += val - that is to say, without self - always assign to the local variable x. In this case that means a value was being assigned to the local variable email (not to be confused with self.email), which was then discarded when the block exited.

¹ See What is attr_accessor in Ruby? for how getters/setters are implemented.

² When ruby sees self.x = y it treats it like self.__send__(:x=, y). That is, the setter (which is just a normal method with a trailing equals in the name) is called magically.

edited May 23 '17 at 10:24

Community

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answered Nov 13 '13 at 05:37

user2864740

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Just to demonstrate one example please... I was not aware of this fact. :) – Arup Rakshit Nov 13 '13 at 05:38
May I add one simple example?.. That I do have to your answer.. :) – Arup Rakshit Nov 13 '13 at 05:49
First demonstrate why the output is `nil`? for the code - `class Foo attr_accessor :x def bar x = 10 end end foo = Foo.new foo.bar p foo.x` – Arup Rakshit Nov 13 '13 at 05:50
Then say what's the fix - `class Foo attr_accessor :x def bar self.x = 10 end end foo = Foo.new foo.bar p foo.x` – Arup Rakshit Nov 13 '13 at 05:51
1

@ArupRakshit You may update my answer as you like. Or add your own. Leaning something new is cool :) – user2864740 Nov 13 '13 at 06:03
Its OK.. I wouldn't create any answer,as your one is enough...people who are interested can see the code in the comment itself.. – Arup Rakshit Nov 13 '13 at 06:04

score 1 · Answer 2 · answered Nov 13 '13 at 05:39

email = email.downcase

Makes a new variable that overwrites the class synthesis of email. My rule of thumb is to always prefix attributes with self. While Ruby allowys you to do either, this is part of the POLS or Principle Of Least Surprise. When you work with other developers it is better if they don't have to dig through your code, and just know that email is an attribute. Bottom line, don't omit self.

And for gods sake, space out your code a tad. Readability is key.

before_save { self.email.downcase! }

You can't have an explicit `self` for a `private` getter, though. — Jörg W Mittag, Nov 13 '13 at 10:38

How to let `self` work in a class

2 Answers2