仅当字符串具有值时才格式化字符串

Here's my code:

这是我的代码:

<a href="/go.html?url=<%= foo_path(foo) %>" class="<%= foo.roles.empty? ? foo.roles.gsub(',', ' ') : ""  %>">

undefined method `empty?' for nil:NilClass
<a href="/go.html?u