# Facebook(er) and Authentication Filters

**URL:** <https://www.ruby-forum.com/t/facebook-er-and-authentication-filters/145232>\
**Category:** Rails\
**Created:** [August 7, 2008, 12:47am UTC](https://www.ruby-forum.com/t/facebook-er-and-authentication-filters/145232 "2008-08-07T00:47:36Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![railsroad](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e274bd/32.png) [@railsroad](https://www.ruby-forum.com/u/railsroad)\
**Post date:** [August 7, 2008, 12:47am UTC](https://www.ruby-forum.com/t/facebook-er-and-authentication-filters/145232/1 "2008-08-07T00:47:36Z")

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I am looking at this app:

> **[Mobtropolis now accessible through facebook](http://mobtropolis.blogspot.com/2008/03/mobtropolis-now-accessible-through.html)**
>
> It took a little bit of code wrangling, but the Mayor pulled a couple of strings, and now you can add Mobtropolis as an app on Facebook ! M...

When in facebook, it knows to force the user to add the app when a link  
which requires it is appropriate, yet when viewing from  
[www.mobtropolis.com](http://www.mobtropolis.com), it knows to send the visitor to  
[http://www.mobtropolis.com/account/login](http://www.mobtropolis.com/account/login).

How does this app do it?!  
How are the filters setup?! Where do they go?