Can XPath do a foreign key lookup across two subtrees of an XML?

Question

Say I have the following XML...

<root>
  <base>
    <tent key="1" color="red"/>
    <tent key="2" color="yellow"/>
    <tent key="3" color="blue"/>
  </base>
  <bucket>
    <tent key="1"/>
    <tent key="3"/>
  </bucket>
</root>

...what would the XPath be that returns that the "bucket" contains "red" and "blue"?

Answer 1

I think this will work:

/root/base/tent[/root/bucket/tent/@key = @key ]/@color

Answer 2

It's not pretty. As with any lookup, you need to use current():

/root/bucket[/root/base/tent[@key = current()/tent/@key]/@color = 'blue' or /root/base/tent[@key = current()/tent/@key]/@color = 'red']

Answer 3

If you're using XSLT, I'd recommend setting up a key:

<xsl:key name="tents" match="base/tent" use="@key" />

You can then get the <tent> within <base> with a particular key using

key('tents', $id)

Then you can do

key('tents', /root/bucket/tent/@key)/@color

or, if $bucket is a particular <bucket> element,

key('tents', $bucket/tent/@key)/@color

Answer 4

JeniT has the appropriate response / code listed here. You need to create the key before you walk the XML Document, then perform matches against that key.