Question

When should I use :conditions or :requirements in rails routing?

Here are two examples:

:conditions

map.connect "/foo/:view/:permalink", :controller => "foo",
    :action => "show", :view => /plain|fancy/,
    :permalink => /[-a-z0-9]+/,
    :conditions => { :method => :get }
end

:requirements

 map.connect 'posts/index/:page',
            :controller => 'posts',
            :action => 'index',
            :requirements => {:page => /\d+/ },
            :page => nil
 end

Answer 1

The only option :conditions takes is :method (i.e. :get, :post, etc.), letting you restrict which methods may be used to access the route:

map.connect 'post/:id', :controller => 'posts', :action => 'show',
            :conditions => { :method => :get }

:requirements, on the other hand, lets you specify a regular expression that the parameter must match, e.g. if the parameter is a postal code you can give it a regular expression that only matches postal codes:

map.geocode 'geocode/:postalcode', :controller => 'geocode',
            :action => 'show', :requirements => { :postalcode => /\d{5}(-\d{4})?/ }

(You can even drop :requirements and use this shorter form:)

map.geocode 'geocode/:postalcode', :controller => 'geocode',
            :action => 'show', :postalcode => /\d{5}(-\d{4})?/

Look under “Route conditions” and “Regular Expressions and parameters” in ActionController::Routing, from which I stole the above examples.