Why isn't self always needed in ruby / rails / activerecord?

In testing a getter/setter pair in a rails model, I've found a good example of behavior I've always thought was odd and inconsistent.

In this example I'm dealing with class Folder < ActiveRecord::Base.

Folder belongs_to :parent, :class_name => 'Folder'

On the getter method, if I use:

def parent_name
  parent.name
end

...or...

def parent_name
  self.parent.name
end

...the result is exactly the same, I get the name of the parent folder. However, in the getter method if I use...

def parent_name=(name)
  parent = self.class.find_by_name(name)
end

... parent becomes nil, but if I use...

def parent_name=(name)
  self.parent = self.class.find_by_name(name)
end

...then then it works.

So, my question is, why do you need to declare self.method sometimes and why can you just use a local variable?

It seems the need for / use of self in ActiveRecord is inconsistent, and I'd like to understand this better so I don't feel like I'm always guessing whether I need to declare self or not. When should you / should you not use self in ActiveRecord开发者_开发百科 models?

This is because attributes/associations are actually methods(getters/setters) and not local variables. When you state "parent = value" Ruby assumes you want to assign the value to the local variable parent.

Somewhere up the stack there's a setter method "def parent=" and to call that you must use "self.parent = " to tell ruby that you actually want to call a setter and not just set a local variable.

When it comes to getters Ruby looks to see if there's a local variable first and if can't find it then it tries to find a method with the same name which is why your getter method works without "self".

In other words it's not the fault of Rails, but it's how Ruby works inherently.