From: Hal Fulton <hal9000@...>
Date: 2005-09-12T14:54:10+09:00
Subject: Re: Misunderstanding of instance variables

Devin Mullins wrote:
> Matthew D Moss wrote:
> 
>> Okay, so the above basically works. But I wondered why the author I
>> borrowed this from used "self." instead of @ -- I thought both would
>> essentially reference an instance variable.
>>
>> So I replaced the one line to look like this:
>>         @content_type = arg.content_type.chomp
>> and it fails, triggering my validation checks.
>>  
>>
> self.content_type = blahblahblah is an invocation of an instance method 
> named "content_type=". In typical implementations of this method*, 
> invocation causes an instance variable @content_type to be set. In the 
> case of ActiveRecord, I guess that's not the case.

Chances are, it does some extra housekeeping in addition to merely
setting the instance variable.

Bear in mind that "x.foo=" is doing a method call. If you defined that
method using attr_writer (or attr_accessor), then it will simply set
the @foo instance variable.

But if you define it manually, it may do anything and everything before
(or after) setting the variable. Or, if you wished to mislead people
or work according to some other model, foo= need not correspond to @foo
at all:

   def foo=(value)
     # Evil and nonsensical
     # Ignore the value
     puts "Haha! Fooled you!"
   end


But in this real-life case, Rails has to do a lot of "bookkeeping" and
I'll bet that's what this is about.


Hal