Ruby precedence of methods in objects extended with multiple modules

Question

Given the following:

class User; attr_accessor :roles; end

module RegisteredUser
  def default_context
    Submission
  end
end

module Admin
  def default_context
    Review
  end
end

current_user = User.new
current_user.roles = ["registered_user", "admin"]
current_user.roles.each do |role|
  role_module = role.gsub(/ /, '_').camelize
  if module_exists?(role_module)
    current_user.extend role_module.constantize
  end
end
context = self.extend current_user.default_context

Is there a way to set the priority of User#default_context? That is, can I say that the Admin#default_context always takes priority over RegisteredUser#default_context regardless of the order in which current_user is extended?

You might be able to do something with `extended` or `method_added`, but you'd need to have some sort of priority rules mechanism either in the modules or somewhere... erm... else. — Dave Newton, Nov 30 '11 at 15:58

Dave Newton · Accepted Answer · 2011-11-30T17:02:20.393

2

method_added is in Module.

I actually meant included, not extended, but both are also in Module.

The mechanism would revolve around doing something like this:

module Foo
  def self.included(base)
    base.extend(FooMethods)
  end

  module FooMethods
    def bar
      # Whatever
    end
  end
end

Inside Foo.included you can determine, based on arbitrary criteria, whether or not the methods in question should be added to base (the entity including the module).

In your case, you could check to see if a "higher priority" module was already included, or see if the module is the "higher priority" module. Based on that you'd decide whether or not to add the methods.

edited Nov 30 '11 at 17:02

answered Nov 30 '11 at 16:38

Dave Newton

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I get "NoMethodError: private method `method_added' called for Foo:Module" when I try Foo.method_added. – Reed G. Law Nov 30 '11 at 17:16
@ReedG.Law That's not how you'd use it, nor am I convinced it's what you'd want to use--I think it's an "after" hook, which doesn't help. `included` would be more appropriate. – Dave Newton Nov 30 '11 at 17:48

score 1 · Answer 2 · answered Nov 30 '11 at 16:31

1

Since an Admin is also a RegisteredUser, I would do

module Admin
  include RegisteredUser
  ...
end

and then only

current_user.extend Admin

I am not sure if this is the correct way. If Admin and RegisteredUser were classes, it would make sense making Admin inherit from RegisteredUser. In case of modules, dunno.

answered Nov 30 '11 at 16:31

Nik O'Lai

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This method seems to work. The only drawback I can see is if there were many modules and complex combinations (i.e. a user who is an admin but not an editor, but Admin includes Editor). – Reed G. Law Nov 30 '11 at 17:08

score 1 · Answer 3 · answered Nov 30 '11 at 16:35

1

You cannot; in Ruby the order of module inclusion is the order in which modules are searched (after the current class, before parent classes). The only way to change the 'priority' is to include the modules in the order you want, or to move them to a parent class.

Although not pure Ruby, you can use the Remix library by banisterfiend to change module ordering (or unmixin a module, or...other things).

answered Nov 30 '11 at 16:35

Phrogz

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I'd have to play, but it sure seems like some hooks into `included`/`extended` could be used to do this, no? – Dave Newton Nov 30 '11 at 16:40
Remix looks good, but it's hard to jump to that kind of solution without further evaluation. Although that kind of functionality could be essential to using the DCI design pattern. – Reed G. Law Nov 30 '11 at 17:27

Ruby precedence of methods in objects extended with multiple modules

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