Use of Instance Variables & self.variable

Question

The ruby gem BCrypt has an example of how to use one of it's modules. Taken from the docs:

def password
  @password ||= Password.new(self.password_hash)
end

def password=(new_password)
  @password = Password.create(new_password)
  self.password_hash = @password
end

Why would we use the instance variable @password at all when we have the self.password_hash attribute? I feel like I am missing something, probably due to my inexperience with ruby. Personally, I would have accomplished what I believe to be the same with:

def password
  self.password_hash ||= Password.new(self.password_hash)
end

def password=(new_password)
  self.password_hash = Password.create(new_password)
end

I started to answer - but then realized, you are right, it is a bit strange. I think we'd have to investigate the surrounding context to figure out what the difference is between @password and #password_hash. I would think your method would work, but there may be other side effects that require these two to be allowed to differ, sometimes. The only thing I can tell for sure is that @password is always a Password instance, but it appears #password_hash can be, but does not appear as if it always would be. Is there an alterate way to call #password_hash=? — Matt
– Matt, Commented Mar 1, 2012 at 22:14

Aleksander Pohl · Accepted Answer · 2012-03-03 13:33:12Z

2

The difference is as follows: password hash is a string - a hash of the password. However Password.new(self.password_hash) creates new object of the Password class. Thus the difference: String vs Password. You can call methods such as cost, version etc. on the Password object, which are unavailable for String.

I guess you find the following method strange:

def password=(new_password)
  @password = Password.create(new_password)
  self.password_hash = @password
end

but what happens here, which is not obvious is the conversion of the Password object to String object in the self.password_hash assignment - the to_s method returns the hash of the password and that value is stored in the database. What is more - the @password instance variable is set, so the password method will return an instance of Password class, not the password hash String.

edited Mar 3, 2012 at 13:33

answered Mar 2, 2012 at 0:29

Aleksander Pohl

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1 Comment

Thomas Nadin Over a year ago

Thank you, that makes perfect sense. looking back at docs, it does state that self.password_hash is a :string in the database. +1

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