Raise exception when accessing attributes that doesn't exist in OpenStruct

Question

I'm trying to figure out how to make it so that a subclass of OpenStruct (or any class for that matter), or hash, will raise a custom exception if I try to access an attribute that hasn't been set. I couldn't get define_method and method_missing to do this so I'm clueless how it should be done in Ruby.

Here's an example:

class Request < OpenStruct...

request = Request.new

begin
  request.non_existent_attr
rescue CustomError...

I could imagine it would have to be something like this:

class Hash
  # if trying to access key:
  # 1) key exists, return key
  # 2) key doesn't exist, raise exception
end

Edit: Attributes that exist shouldn't raise an exception. The functionality I'm looking for is so that I can just access attributes freely and if it happens not to exist my custom exception will be raised.

Answer 1

I use something like

hash = { a: 2, b: 3 }

Struct.new(*hash.keys).new(*hash.values).freeze

to get an immutable object which will raise NoMethodError in case unexpected method is invoked

Answer 2

If you need a strict hash, simply:

class StrictHash < Hash
  alias [] fetch
end

It works as expected:

hash = StrictHash[foo: "bar"]

hash[:foo]
# => "bar"

hash[:qux]
# stricthash.rb:7:in `fetch': key not found: :qux (KeyError)
#         from stricthash.rb:7:in `<main>'