Xpath - How to navigate to a value (Ruby Nokogiri) - ruby

Question

If I want to grab a currencies rate, say "USD", given a certain time, say "2015-02-09", how would I go about doing this?
I tried the following:
/gesmes:Envelope/def:Cube/def:Cube[#time="2014-11-19"]/def:Cube[#currency="USD"]/#rate
Though I suppose due a lack of understanding this is wrong, well at least, I know it is wrong because Nokogiri does not run it.
http://www.ecb.europa.eu/stats/eurofxref/eurofxref-hist-90d.xml
EDIT:
I'm going to go ahead and guess that I am not correctly using Nokogiri and XPath.
#doc = Nokogiri::XML(File.open("exchange_data.xml"))
#values = #doc.xpath('XPATH HERE')
#values.each {|i| puts i}
I have read the tutorial, and managed to get it working for other xml files, but this one seems harder to crack.

Answer 1

require 'nokogiri'
doc = Nokogiri::XML(File.open("xml4.xml"))
target_date = "2015-02-09"
target_currency = 'USD'
xpaths = [
"//gesmes:Envelope",
"/xmlns:Cube",
"/xmlns:Cube[#time='#{target_date}']",
"/xmlns:Cube[#currency='#{target_currency}']",
]
xpath = xpaths.join
target_cube = doc.at_xpath(xpath)
puts target_cube.attribute('rate')
--output:--
1.1297
Response to comment:
Your root tag:
<gesmes:Envelope xmlns:gesmes="http://www.gesmes.org/xml/2002-08-01"
xmlns="http://www.ecb.int/vocabulary/2002-08-01/eurofxref">
...declares two namespaces with xmlns, which stands for xml namespace. The namespace:
xmlns:gesmes="http://www.gesmes.org/xml/2002-08-01"
declares that any child tag whose name is prefixed by gesmes, e.g.:
<gesmes:subject>
...
</gesmes:subject>
will actually have a tag name that incorporates the specified url into the tag name, something like this:
<http://www.gesmes.org/xml/2002-08-01:subject>
...
</http://www.gesmes.org/xml/2002-08-01:subject>
The reason you would want to use a namespace is to create a unique name for the Cube tag, so that it doesn't clash with another xml document's Cube tag.
The second namespace declaration:
xmlns="http://www.ecb.int/vocabulary/2002-08-01/eurofxref"
is a default namespace declaration. It declares that any child tag that does not specify a prefix will have the specified url incorporated into its tag name. So a tag like this:
<Cube>
...
</Cube>
becomes something like this:
<http://www.ecb.int/vocabulary/2002-08-01/eurofxref:Cube>
...
</http://www.ecb.int/vocabulary/2002-08-01/eurofxref:Cube>
However, it would be unwieldy to have to write a tag name like that in your xpaths, so in place of the url you instead use the shortcut xmlns:
/xmlns:Cube

Answer 2

This might be due to the namespaces in this document:
<gesmes:Envelope xmlns:gesmes="http://www.gesmes.org/xml/2002-08-01" xmlns="http://www.ecb.int/vocabulary/2002-08-01/eurofxref">
To test this hypothesis, apply the following XPath expression:
/*[local-name() = 'Envelope']/*[local-name() = 'Cube']/*[local-name() = 'Cube'][#time="2014-11-19"]/*[local-name() = 'Cube'][#currency="USD"]/#rate
and let me know what you get. If you are otherwise correctly using XPath, you should end up with:
rate="1.2535"
If not, you are not using the XPath facilities of Nokogiri correctly, and then you'd really need to show all of your Ruby code to get help.
EDIT
Responding to a comment:
I look forward to seeing some examples added to your answer, so that I can learn something new about xml namespaces. – 7stud
7stud already gave the correct answer, I'll only add info I think is missing from this answer.
Explicit namespaces
First of all, if a namespace URI is explicitly present on an element, the correct syntax uses curly brackets, both for a prefixed and default namespace:
<{http://www.gesmes.org/xml/2002-08-01}subject>
Internally, this is how namespaces could be represented on elements (although some applications have other ways to associate elements with namespaces). Prefixes and default namespaces are there to simplify this process.
Namespaces in Nokogiri
Prefixes (gesmes:) do not have any inherent meaning. They can be associated with an arbitrary namespace URI and every document can use gesmes: to mean something different. Namespace declarations are not available to an XPath engine per se - usually, if you'd like to use a prefix in an XPath expression, you need to declare this namespace again for the XPath processor.
Yet, Nokogiri tries to simplify namespace handling for you by redeclaring namespace declarations found on the root element of the input document. This is important because it allows you to reuse the prefixes declared on the root element of the input without actually declaring the namespace. For default namespaces declared on the root element that do not have a prefix, Nokogiri has defined a special syntax:
xmlns:Cube
Namespaces that are present in the document, but declared on an element other than the root element:
<root>
<child xmlns:gesmes="http://other.com"/>
</root>
must be explicitly declared in Nokogiri:
#doc.xpath('//other:Cube', 'other' => 'http://other.com/')
What's wrong with your original code?
Your code:
/gesmes:Envelope/def:Cube/def:Cube[#time="2014-11-19"]/def:Cube[#currency="USD"]/#rate
does not work because you are using an unknown prefix def:. This prefix is not declared on the root element of the input, and neither did you declare it with Nokogiri. The Cube elements are in the default namespace, and, as we have seen, the correct way to address them is
/gesmes:Envelope/xmlns:Cube
and so on, 7stud gave you the correct answer.