问题描述
我试图在xml文件中为一个特定的元素设置一个属性，我使用
I was trying to set an attribute for a specific element in an xml file and I was having success using
doc.css('Object').attr("Id").value = timestamp
对象不存在导致程序中的异常并退出的情况。为了避免我想使用Nodeset，因为它只是空的。
This was fine until the situation where 'Object' doesn't exist causing an exception in the program and quitting. To avoid that I wanted to use Nodeset as it'll just be empty instead.
doc.css('Object').each do |element|
element.attr("Id").value = timestamp
end
但是，这返回一个错误，值=是一个未定义的方法。这可能是简单的，但我是新的Ruby和CSS所以任何帮助将是伟大的。
However this returns with the error that value= is an undefined method. It's probably something simple but I'm new to Ruby and CSS so any help would be great.
推荐答案
这个问题与CSS无关，因为Nokogiri使用CSS选择器来替代使用XPath选择器，只用于提供到一个节点或节点的路径。
The problem has little to do with CSS, since Nokogiri uses CSS selectors as an alternative to using XPath selectors, and both are only used to provide a path to a node, or nodes.
看起来你正在考虑这个问题，并使它比它需要的更难。这是我要做的：
It looks like you're overthinking this, and making it harder than it needs to be. Here's what I'd do:
require 'nokogiri'
doc = Nokogiri::XML(<<EOT)
<xml>
<foo>
<Object>bar</Object>
</foo>
</xml>
EOT
doc.at('Object')["Id"] = Time.now.to_s
查看 doc 此时显示：
puts doc.to_xml
# >> <?xml version="1.0"?>
# >> <xml>
# >> <foo>
# >> <Object Id="2014-01-28 19:13:32 -0700">bar</Object>
# >> </foo>
# >> </xml>
了解，和，它返回第一个匹配的和，和，它们返回NodeSets。 类似于包含节点的数组。当你知道，你的声明：
It's really important to understand the difference between at, at_css and at_xpath, which return the first matching Node, and search, css and xpath, which return NodeSets. A NodeSet is akin to an array containing Nodes. When you know that, your statement:
doc.css('Object').attr("Id").value = timestamp
没有什么意义，特别是因为这不是方法定义： p>
won't make much sense, especially since that's not how the attr method is defined:
attr(key, value = nil, &blk)
您需要使用：
doc.css('Object').attr("Id", value)
< Object> 节点的所有 Id 属性
which would assign value to all Id attributes for every <Object> node in the document.
但是，再次，这不是正确的选择，而应该使用在或 at_css 返回单个节点。
But, again, that's not the right choice, instead you should use at or at_css to return the single node.
如果没有< Object> 节点存在， ，你必须决定做什么。
If no <Object> node exists, then it gets more interesting, and you have to determine what to do. You can insert it, or, you can simply move along and do nothing.
要查看节点是否存在很简单：
To see if a node exists is simple:
object_node = doc.at('Object')
if object_node
object_node['Id'] = Time.now.to_s
else
# ... insert it
end
找到要插入它的位置，然后添加它：
To insert a node involves locating the place you want to insert it, then add it:
doc.at('foo').add_child("<Object Id='#{ Time.now }'>baz</Object>")
puts doc.to_xml
# >> <?xml version="1.0"?>
# >> <xml>
# >> <foo>
# >> <Object>bar</Object>
# >> <Object Id="2014-01-28 19:39:38 -0700">baz</Object></foo>
# >> </xml>
我没有尝试使XML输出漂亮，这在XML中并不重要，
I didn't try to make the XML output pretty, which isn't important in XML, it merely needs to be syntactically correct.
还要注意，可以通过将一个节点或多个节点定义为一个XML字符串来插入这些节点。 Nokogiri会把它解析成合适的XML并移植到你说的地方。你也可以通过定义一个NodeSet或Node，然后插入它，走长路由，但是，一般来说，这使得更丑陋的代码，并导致你做更多的工作，这导致较少可读的代码为那些跟随你的脚步保持源。
Also note that it's possible to insert a node, or nodes, by defining them as a string of XML. Nokogiri will parse it into the appropriate XML and graft it in where you said. You could also go the long route by define a NodeSet or Node, then inserting it, but, in general, that makes uglier code and causes you to do a lot more work, which results in less readable code for those who follow in your footsteps maintaining the source.
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