Nokogiri equivalent of jQuery closest() method for finding first matching ancestor in tree

Question

jQuery has a lovely if somewhat misnamed method called closest() that walks up the DOM tree looking for a matching element. For example, if I've got this HTML:

<table src="foo">
  <tr>
    <td>Yay</td>
  </tr>
</table>

Assuming element is set to <td>, then I can figure the value of src like this:

element.closest('table')['src']

And that will cleanly return "undefined" if either of the table element or its src attribute are missing.

Having gotten used to this in Javascriptland, I'd love to find something equivalent for Nokogiri in Rubyland, but the closest I've been able to come up with is this distinctly inelegant hack using ancestors():

ancestors = element.ancestors('table')
src = ancestors.any? ? first['src'] : nil

The ternary is needed because first returns nil if called on an empty array. Better ideas?

Answer 1

You can call first on an empty array, the problem is that it will return nil and you can't say nil['src'] without getting sad. You could do this:

src = (element.ancestors('table').first || { })['src']

And if you're in Rails, you could use try thusly:

src = element.ancestors('table').first.try(:fetch, 'src')

If you're doing this sort of thing a lot then hide the ugliness in a method:

def closest_attr_from(e, selector, attr)
  a = e.closest(selector)
  a ? a[attr] : nil
end

and then

src = closest_attr_from(element, 'table', 'src')

You could also patch it right into Nokogiri::XML::Node (but I wouldn't recommend it):

class Nokogiri::XML::Node
  def closest(selector)
    ancestors(selector).first
  end
  def closest_attr(selector, attr)
    a = closest(selector)
    a ? a[attr] : nil
  end
end

Answer 2

You can also do this with xpath:

element.xpath('./ancestor::table[1]')

Answer 3

You want the src attribute of the closest table ancestor, if it exists? Instead of getting an element that might exist via XPath and then maybe getting the attribute via Ruby, ask for the attribute directly in XPath:

./ancestor::table[1]/@src

You'll get either the attribute or nil:

irb(main):001:0> require 'nokogiri'
#=> true

irb(main):002:0> xml = '<r><a/><table src="foo"><tr><td /></tr></table></r>'
#=> "<r><a/><table src=\"foo\"><tr><td /></tr></table></r>"

irb(main):003:0> doc = Nokogiri.XML(xml)
#=> #<Nokogiri::XML::Document:0x195f66c name="document" children=…

irb(main):004:0> doc.at('td').at_xpath( './ancestor::table[1]/@src' )
#=> #<Nokogiri::XML::Attr:0x195f1bc name="src" value="foo">

irb(main):005:0> doc.at('a').at_xpath( './ancestor::table[1]/@src' )
#=> nil