Apparently code order matters in Ruby

I was recently working on some code that had a structure like this:

module OuterModule
  class MyObject
    include InnerModule
  end

  module InnerModule
    def hello
      "hello"
    end
  end
end

include OuterModule

m = MyObject.new

puts m.hello

When I ran it, I would get this unfortunate output:

test_ruby_class_heirarchy.rb:3: uninitialized constant OuterModule::MyObject::InnerModule (NameError)

I'm too embarrased to list all the voodoo I tried to get this to work, but simply putting the definition of MyObject below that of InnerModule did the trick:

module OuterModule
  module InnerModule
    def hello
      "hello"
    end
  end

  class MyObject
    include InnerModule
  end
end

include OuterModule

m = MyObject.new

puts m.hello

This gives me the expected output:

hello

Going into this, I would have never guessed that the order in which I defined things would have this effect.