Ruby - Share local variable with the obj's eigenclass

I am trying to dynamically define methods on an initialized Ruby object MyObject based on a hash my_hash I pass to its initialize method. In the body of the initialize method, I have the following:

my_hash.each do |key|
  class << self
    define_method(key.underscore.to_sym) do
      my_hash[key]
    end
  end
end

This fails with a undefined local variable or method 'key' for #<Class:#<MyObject:0x007fc7abw0cra0>>. Any ideas why?

The my_hash is made out of a json response with a lot of camelized keys, so it's more convenient to have simple ruby methods to get the values I want.

Local variables are local to the scope they are defined in. The (lexical) scopes in Ruby are script scope, module/class definition scope, method definition scope and block scope. Only block scopes nest in their outer scopes (aka close over their surrounding scope). So, you have to use a block:

my_hash.each_key do |key|
  singleton_class.class_eval do
    define_method(key.to_sym) do
      my_hash[key]
    end
  end
end

Or even better yet:

my_hash.each_key do |key|
  define_singleton_method(key.underscore.to_sym) do
    my_hash[key]
  end
end

Note: I also fixed a bug, you should be iterating the keys with each_key, not each (which is an alias for each_pair).