为了实现 Comments 的 Ajax 提交，为 create 方法添加了 create.js.erb 来实现：
其中在 append 的渲染结果时，我需要引入当前总评论数
A 写法 (成功)
$('#comments').append('<%= escape_javascript(render @comment, {:comment_counter => @comments_size} ) %>');
B 写法（失败，由于 comment_counter 未定义）
$('#comments').append('<%= escape_javascript(render @comment, :locals => {:comment_counter => @comments_size } ) %>');
我想了解的是，为什么加了 locals 却不行呢？
def render(options = {}, locals = {}, &block)
case options
when Hash
if block_given?
view_renderer.render_partial(self, options.merge(:partial => options[:layout]), &block)
else
view_renderer.render(self, options)
end
else
view_renderer.render_partial(self, :partial => options, :locals => locals)
end
end
如果第一个参数不是 Hash 的话，将会执行
view_renderer.render_partial(self, :partial => options, :locals => locals)
render @comment, :locals => {:comment_counter => @comments_size} 这样将会 :locals 参数将会被忽略掉
所以你应该
render(:partial => @comment, :locals => {:comment_counter => @comments_size})
# 或者
render(@comment, {:comment_counter => @comments_size})