content_for vs yield in partials

问题

In rails 3.0 with HAML (3.1.4) I have

1. some template-like partial, like _template.html.haml:

   .panel.top
     = yield :panel_top
   
   .content
     = yield
   

2. some another partial which will be displayed using prev template (all this stuff is rendered using AJAX, but this doesn't matter)

   - content_for :panel_top do
    .title.left
      = title
   
   content text
   

and this worked like a charm in Rails 3.0

But, after upgrade to 3.2 this fails! Yiels just yields "content text", so I have "content text" twice and no title at all

only changing = yield :panel_top to = content_for :panel_top works for 3.2

I am not sure that this solution is ok, and if it is stable or recommended, I cannot find any notes about changes in yield processing nor in Rails 3.1 release notes, nor in 3.2 ones.

Can you help what is the best way to organize yielding inside partials?

回答1:

From Rails 3.0 to Rails 3.2 content_for was really changed:

3.0:

def content_for(name, content = nil, &block)
    content = capture(&block) if block_given?
    @_content_for[name] << content if content
    @_content_for[name] unless content
end

3.2:

def content_for(name, content = nil, &block)
  if content || block_given?
    content = capture(&block) if block_given?
    @view_flow.append(name, content) if content
    nil
  else
    @view_flow.get(name)
  end
end

This shows us, that from 3.2 content_for works for showing/inserting content too, not only store it for named section.

Also, if you make an attempt to debug yield logic you'll se that it yields before content_for is correctly initialized.

So, leaving fragment caching out of this discussion I can conclude that content_for is preferrable way to insert named sections anywhere except top-level layouts. In helpers and other situations yield should render wrong results.

来源：https://stackoverflow.com/questions/9185344/content-for-vs-yield-in-partials