Module.new with class_eval

This is a large commit. But I want you to concentrate on this change block. http://github.com/rails/rails/commit/d916c62cfc7c59ab6411407a05b946d3dd7535e9#L2L1304

Even without understanding the full context of the code I am not able to think of a scenario where I would use

include Module.new {
  class_eval <<-RUBY
    def foo
       puts 'foo'
    end
  RUBY
}

Then end result is that in the root context (self just before include Module.new) a method called foo has been added.

If I take out the Module.new code and if I only leave class_eval in that case also I will have a method called foo in self.

W开发者_运维知识库hat am I missing.

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If you dig in the documentation you find that including a module will add the methods therein only if they are not already defined. So this approach will not overwrite the method in case it is already there.

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This ActiveRecord code has been asked about in another question, where it received an excellent answer. https://stackoverflow.com/a/3473479/420947

However, the simplified eval string here removes the motivation to write this code, which is why it appears confusing. In the unchanged code, the block binding captures a local variable used to reflect on the association: #{reflection.name}.clear.