Ruby on Rails

On Dec 1, 9:16 pm, Michael Pavling <pavl...@gmail.com> wrote:
> On 1 December 2011 20:57, Alexey Muranov <li...@ruby-forum.com> wrote:
>
> > I tried to replace
>
> > self.fixture_class_names = self.fixture_class_names.merge(class_names)
>
> > with
>
> > self.fixture_class_names.merge!(class_names) # Why does this fail?
>
> 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

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