[Rails] Re: hash.merge!(other) vs hash = hash.merge(other)

Frederick Cheung wrote in post #1034631:
> On Dec 1, 9:16pm, Michael Pavling <pavl...@gmail.com> wrote:
>> At a guess, I'd say because "self.fixture_class_names=" is a method
>> that does some setting within it, while with the other syntax, you're
>> trying to call a bang method on the return value of a method.
>
> Getting very warm! self.fixture_class_names is (or at least was -
> haven't checked rails 3.1) a superclass_delegating_accessor.
> This means that when you call it on a subclass it travels up the
> inheritance hierarchy looking for a class where it has been set.
> So if we had classes A < B < C < D, and A.fixture_class_names = x then
> calling D.fixture_class_names checks D, then C, then B a lastly A
> where it stops, since a value has been defined for A.
> When you call C.fixture_class_names = y that doesn't change what B
> does (it checks B and then A), but it changes what D does (it checks
> D, then C and returns y).
> However, if you do C.fixture_class_names.merge! (without having
> called C.fixture_class_names=) that will change the value that
> 'belongs' to A, thus changing what A.fixture_class_names and what
> B.fixture_class_names returns
>
> Fred

Thanks for the explanation. fixture_class_names is in fact declared
with

class_attribute :fixture_class_names

Didn't you mean D < C < B < A ?

Alexey.

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