You've set up rails with devise and omniauth. Users are signing up with facebook or twitter or email&password or something completely different. Everything is going great.

Now you want to force some users to add a facebook login to their account. Maybe you just want to see if their name and avatar match what they've set up in your service to make sure they really are who they say they are. Whatever.

It seems like you might need to add a custom Devise strategy. You start googling. Doesn't seem like a lot of work, you just extend Authorizable and override the authorize! function and then ... do something. Seems you'll have to mostly override how authorization works. Will that play well with omniauthable?

But that only solves the problem of forcing Facebook on authorization. Who's going to check the conditions on every page load and sign the user out?

You set up a filter. Put it in application_controller and it's just a big mess. You're still not sure how to tie everything together and the clusterfuck of code is growing and growing.

https://twitter.com/Swizec/statuses/347024137086173185

Force FB with just 19 sloc

There's a better way!

I stumbled on this solution almost by accident after ending up on an obscure API doc page on my gazillionth google search. Turns out you can force a user to sign up with facebook in just under 19 lines of code.

We have to extend how models behave, not create a new Devise strategy. The function we're looking to override is active_for_authentication?.

This function is called on every page load when a user is signed in and checks that they're still active. If it returns false the user will be signed out and asked to login before continuing. They won't be let through until the function returns true.

Elegant!

All you have to do is put something like this in your app/classes directory.

module Facebookable
  extend ActiveSupport::Concern
  def needs_facebook?
    needs_facebook && services.where("provider = 'Facebook'").count < 1
  end
  def should_flag?
    condition # get a score somehow
    if condition
      self.needs_facebook = true
      self.save!
    end
  end
  def active_for_authentication?
    super && !needs_facebook? && !should_flag?()
  end
  def inactive_message
    needs_facebook? ? "Please sign in with Facebook." : super
  end
end

And that's essentially it. Add a boolean needs_facebook field to your user model and add :facebookable to the strategies list and you're done. Everything works.

Facebookable will automatically take care of checking whether a user has facebook connected and kick them out if they don't but should. On every page load it will also check whatever condition you've set and flag the user.

It took me four days to come up with those 19 lines of code because nobody's written a blogpost about how to do this properly. Now someone has.

PS: testing this works the same as testing any other model methods.

Have a burning question that you think I can answer? I don't have all of the answers, but I have some! Hit me up on twitter or book a 30min ama for in-depth help.

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