hash.merge!(other) vs hash = hash.merge(other)

rubyonrails-talk
1

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?

in rails source, and it failed 2 tests.

What is the difference between the two? Could it have something to do with the fact that the fixture_class_names hash has a block to generate values that were not assigned? Or is this because fixture_class_names is a class attribute?

2

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.

3

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

4

Frederick Cheung wrote in post #1034631:

5

Frederick Cheung wrote in post #1034631:

class_attribute :fixture_class_names

The implementation is a bit different but the no-no on mutating the blah.fixture_class_names is the same

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

yes :slight_smile:

A simpler example in which merge! and merge= wouldn't be the same would be

class Foo   def self.bar     @bar.dup #prevent people mutating me!   end

  def self.bar=(value)     @bar = value   end end

Clearly Foo.bar.merge!(...) will not accomplish much here

Fred