XPath to find nearest ancestor element that contains an element that has an attribute with a certain value

Question

ancestor::foo[bar[@attr="val"]]

I thought this would work, but it's not. I need to find the nearest foo element going up in the tree that has a bar child element, which in turn has an attr attribute of val.

Solution 1

This should work:

//*[ancestor::foo[bar[@attr="val"]]]

or alternatively

root/foo/bar[ancestor::foo[bar[@attr="val"]]]

matches the second <bar> element

<root>
    <foo>
        <bar attr="xxx"></bar>
    </foo>
    <foo>
        <bar attr="val"></bar>
    </foo>
    <foo>
        <bar attr="zzz"></bar>
    </foo>
</root>

Solution 2

Updated to reflect a solution intended by OP.

See @David's answer

For a sample xml below,

<root>
<foo id="0">
    <bar attr="val"/>
    <foo id="1">
        <bar attr="xxx"/>
    </foo>
    <foo id="2">
        <bar attr="val">
            <one>1</one>
            <two>
                <three>3</three>
            </two>
        </bar>
    </foo>
</foo>

a valid xpath on the reverse axis with <three> as the context node, a valid solution is

./ancestor::foo[bar[@attr='val']][position() = 1]

"./" is optional. position() = 1 is not optional as otherwise the ancestor foo satisfying the predicates would also be returned.

Old answer:ignore

This is an old question. but anybody who is interested, the following should get you what the original question intended.

Reverse axis

//bar[@attr='val']/ancestor::*[position()=1]

Forward axis

//*[child::bar[@attr='val']]

Solution 3

In my opinion the best answer was provided by the person who posed the question. His solution was supposed to return all the ancestor foos. He wants only the nearest one which is the one with position()=1, so his xpath expression needs to be slightly amended to:

ancestor::foo[bar[@attr="val"] and position() = 1]

If he writes that the ancestor::foo[bar[@attr="val"]] didn't return anything, so he had some other problem in his xml or in his assumption about the current element of his XPath evaluation context.

The other answers (starting with //) do not really answer the question and even if they by chance met the need of someone, so they are not efficient: they choose ALL elements in the xml file, applying filter on them afterwards. While what the relative xpath proposed by me or the person who was asking this question is just going to search through few elements starting from the current node up to the parent and it's parent etc. - it will be very efficient even with large XML files.

Solution 4

If you have a file like this:

<root>                                                                                                             
<foo id="0">                                                                                                       
    <foo id="1">                                                                                                   
        <bar attr="xxx" />                                                                                         
    </foo>                                                                                                         
    <foo id="2">                                                                                                   
        <bar attr="val" />                                                                                         
    </foo>                                                                                                         
    <foo id="3">                                                                                                   
        <tar>                                                                                                      
            <bar attr="val" />                                                                                     
        </tar>                                                                                                     
    </foo>                                                                                                         
</foo>                                                                                                             
</root>

and you want the foo nodes with the ids 2 and 3, i.e. the closest foos to their bar descendants having attr=val, the following works:

//foo[descendant::bar[@attr="val"] and not(descendant::foo)]

If you omit and not(descendant::foo) you get additionally the foo with the id 0.

The other answers didn't work for me on the general case of my example.

You can test it in Python with:

from lxml import etree
tree = etree.parse('example.xml')
foos = tree.xpath('//foo[descendant::bar[@attr="val"] and not(descendant::foo)]')
for foo in foos:
    print(foo.attrib)

which prints:

{'id': '2'}
{'id': '3'}

Notice that the xpath used is not efficient. I would love to learn a more efficient one.

View more solutions

Comments

[Simon Green]

Does this actually answer the question? The OP wants to select the ancestor (the foo).. this example selects the descendant (the bar) whose ancestor matches some criteria.

[geoidesic]

This doesn't answer the question. Not sure why it has been up-voted.

[Gordon]

@geoidesic because it apparently solved the OPs problem

[wybe]

The reverse of "ancestor" is "descendant", not "child". "child" is the reverse of "parent" instead of "ancestor"

[santon]

@wybe I think you are confusing xpath axes directions with antonyms of an axis. Any axis which finds nodes in document order after the context node is a forward axis. An axis that finds nodes in document order before context node is a reverse axis. If that does not address what you intended, can you please provide what you really meant by that .

[santon]

I agree with your assessment. My answer addressed the problem from top down and does not address it from the context node. I will update my answer to reflect that. thx

[santon]

@wybe Refer to w3.org/TR/xpath/#axes and under section 2.4 Predicates

[wybe]

I meant "the opposite of ancestor is descendant" as in "the opposite of a parent's parent is a child's child". And similarly the other direction of "direct parent" is "direct child". Thanks for clearing this up

[santon]

@wybe I agree with your statements but was confused about the context for that statement. child, descendent, following-sibling, following, descendent-or-self are all forward axis.

[Krzysztof Walczewski]

It's nice and easy to remember!

[Nebulosar]

But not the question :)

[Kyle]

Just because this is the best answer chosen by OP doesn't mean it's correct