Rails 3 vs 2.3.5 render oddities with :partial and :layout - ruby-on-rails

Question

I've come across an oddity that I can't quite explain with regards to Rails 3 and rendering partials with layouts (from the controller). I'm hoping someone can provide a little insight into what's happening.
First off, we'll call this controller a "legacy" controller. It's been around for a long time and is doing a lot of things wrong, but I'm not looking to refactor it at this point so I'm trying to find ways to work with what we have.
The new action is something like this (in the BarsController)
def new
if something
render :partial => "foo", :layout => "bars"
elsif something_else
render :partial => "foo2", :layout => "bars"
elsif something_else_else
render :partial => "foo3", :layout => "bars"
else
render :partial => "foo4", :layout => "bars"
end
Now, in Rails 2.3.5, this worked fine. It would render the appropriate partial inside the appropriate layout -- I realize the layout option is redundant here as it would default to the bars layout regardless. When we upgraded to Rails 3.0.x, we started getting errors as follows:
Missing partial layouts/bars with {:handlers=>[:erb, :rjs, :builder, :rhtml, :rxml], :formats=>[:html]
Clearly the layouts/bars.html.erb file is and always has been there, so I couldn't figure it out. I was able to render with :layout => false, but that of course wasn't going to work. Eventually I figured out that if I do either of the following, it works:
1) Rename my layout to _bars.html.erb instead of bars.html.erb and:
render :partial => 'foo2', :layout => 'bars'
2) Keep my layout as bars.html.erb (what I want) and :
render '_foo2' # :partial option is redundant here anyway
It seems as though by using the :partial option instead of the string as first parameter is causing rails to apply the _name.html.erb convention to both the partial AND the layout. If I put in the underscore on my own, it falls back to the behaviour I expected which is to not prepend an _ infront of the name of the layout.
Does anyone know why this is the case?
EDIT Alright, not sure how I missed this... but here's something in the docs making mention of this. It seems as though it's been around since 2.3.8, perhaps it was done differently in 2.3.5 (what we were running on)?
3.4.3 Partial Layouts
A partial can use its own layout file, just as a view can use a
layout. For example, you might call a partial like this:
<%= render "link_area", :layout => "graybar" %> This would look for a
partial named _link_area.html.erb and render it using the layout
_graybar.html.erb. Note that layouts for partials follow the same
leading-underscore naming as regular partials, and are placed in the
same folder with the partial that they belong to (not in the master
layouts folder).

Answer 1

Here's my own answer to the question based on what I've edited above:
Since Rails 2.3.8, it would appear as though the default behaviour when rendering a partial with render :partial => 'foo', :layout => 'bars' is to expect a "partial layout" file as well as a partial view file. In this case it will expect
app/views/_foo.html.erb as well as app/views/layouts/_bars.html.erb
For anyone encountering this problem upgrading from Rails 2.3.5, here's the solution I found to have the least amount of impact:
render '_foo', :layout => 'bars'
This solution does not assume you're rendering a partial and therefore does not expect a partial layout. The other option would be to duplicate your layout to
app/views/layouts/_bars.html.erb
and using
render :partial => 'foo', :layouts => 'bars'
but that results in some duplication of code.
RAILS 2.3.8+ DOC REGARDING THIS:
3.4.3 Partial Layouts
A partial can use its own layout file, just as a view can use a
layout. For example, you might call a partial like this:
<%= render "link_area", :layout => "graybar" %> This would look for a
partial named _link_area.html.erb and render it using the layout
_graybar.html.erb. Note that layouts for partials follow the same
leading-underscore naming as regular partials, and are placed in the
same folder with the partial that they belong to (not in the master
layouts folder).