Partials and a question of correct form

I'm new to Rails and trying to figure out the correct way to handle
dynamic data in a partial. The partial in question will be used to
display a google ad. The html for this ad will differ based upon the
user (do they pay to not see ads, what the format/style of the ad will
look like needs to match the look and feel they have configured for
their profile, etc.).

I was looking to place an ad in a view as follows:

<%= render "shared/ad_banner", :ad_type => "120x90"%>

where the ad type basically specifies what size ad the ad_banner
partial needs to render. The different html for the ads are stored in
a database. My question is can/should the partial do a lookup to
figure out what ad html to render based upon the ad_type passed in
(along with information about the currently logged in user)? Or should
the actual lookup occur in the controller and the html be passed into
the partial? My first thought was it would be cleaner if I had the
partial do the lookup since the controller would not have to have any
knowledge about what size ad needs to be rendered, this information
was basically embedded in the view.

However, when trying to figure out how to make the partial do the
lookup I started wondering if this was bad form based upon the lack of
examples I saw for this type of approach (all the examples seemed to
be passing the data in).

Any guidance would greatly be appreciated.


My thought would be that the view code should preferably not be
looking up stuff. The controller should provide (via @ variables) all
the data that it needs. However, if you are talking about the size of
something on display this should certainly not be defined in the
controller. The controller should not say 'display this at a certain
size', but should provide whatever data the view needs in order that
it can decide the size. In fact generally the size would be in css
with the view just setting up classes so even the view does not have
to know the size or layout.


you should keep logic out of views partials.

here are a couple different ways to accomplish this

1) create a helper function

# app/helpers/application_helper.rb
def render_ad_html
  if not logged_in? or current_user.show_ads?
    ad = Ad.find_by_controller_name( params[:controller] ) #
    render :partial => "shared/ad_#{ad.dimensions}", :ad => ad

then in your application layout file (or in any view) you can call
<%= render_ad_html %>

2) create a before_filter (*preferred)
# app/controllers/application_controller.rb
class ApplicationController < ...
  def setup_html_ads
     if logged_in? and current_user.show_ads?
        @ad = Ad.find_by_controller_name( params[:controller] )

you can load the before_filter in whatever controllers you want to
show ads
class UserController < ApplicationController
  before_filter :setup_html_ads

and render them in your views if you have an @ad object
<%= render :partial => "shared/ad" if @ad %>

either way will work. but it's best to keep your views as dumb as
possible. for instance, if you want to show the adverts in your rss
stream or in emails...
you don't want to worry about the conditional logic contained in the
partial... since you just want the html.