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

Answer 1

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.