XPath/XSLT nested predicates: how to get the context of outer predicate?

Question

It seems that this question was not discussed on stackoverflow before, save for Working With Nested XPath Predicates ... Refined where the solution not involving nested predicates was offered.

So I tried to write the oversimplified sample of what I'd like to get:

Input:

<root>
    <shortOfSupply>
        <food animal="doggie"/>
        <food animal="horse"/>
    </shortOfSupply>
    <animalsDictionary>
        <cage name="A" animal="kittie"/>
        <cage name="B" animal="dog"/>
        <cage name="C" animal="cow"/>
        <cage name="D" animal="zebra"/>
    </animals>
</root>

Output:

<root>
    <hungryAnimals>
        <cage name="B"/>
        <cage name="D"/>
    </hungryAnimals>
</root>

or, alternatively, if there is no intersections,

<root>
    <everythingIsFine/>
</root>

And i want to get it using a nested predicates:

<xsl:template match="cage">
    <cage>
        <xsl:attribute name="name">
            <xsl:value-of select="@name"/>
        </xsl:attribute>
    </cage>
</xsl:template>

<xsl:template match="/root/animalsDictionary">
    <xsl:choose>
        <!--                                                             in <food>     in <cage>       -->
        <xsl:when test="cage[/root/shortOfSupply/food[ext:isEqualAnimals(./@animal, ?????/@animal)]]">
            <hungryAnimals>
                <xsl:apply-templates select="cage[/root/shortOfSupply/food[ext:isEqualAnimals(@animal, ?????/@animal)]]"/>
            </hungryAnimals>
        </xsl:when>
        <xsl:otherwise>
            <everythingIsFine/>
        </xsl:otherwise>
    </xsl:choose>
</xsl:template>

So what should i write in place of that ??????

I know i could rewrite the entire stylesheet using one more template and extensive usage of variables/params, but it makes even this stylesheet significantly more complex, let alone the real stylesheet i have for real problem.

It is written in XPath reference that the dot . sign means the current context node, but it doesn't tell whether there is any possibility to get the node of context before that; and i just can't believe XPath is missing this obvious feature.

Answer 1

XPath 2.0 one-liner:

for $a in /*/animalsDictionary/cage
      return
        if(/*/shortOfSupply/*[my:isA($a/@animal, @animal)])
          then $a
          else ()

When applied on the provided XML document selects:

   <cage name="B"/>
   <cage name="D"/>

One cannot use a single XPath 1.0 expression to find that a given cage contains a hungry animal.

Here is an XSLT solution (XSLT 2.0 is used only to avoid using an extension function for the comparison -- in an XSLT 1.0 solution one will use an extension function for the comparison and the xxx:node-set() extension to test if the RTF produced by applying templates in the body of the variable contains any child element):

<xsl:stylesheet version="2.0"
 xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
 xmlns:xs="http://www.w3.org/2001/XMLSchema"
 xmlns:my="my:my" exclude-result-prefixes="xs my">
 <xsl:output omit-xml-declaration="yes" indent="yes"/>

 <my:Dict>
  <a genName="doggie">
    <name>dog</name>
    <name>bulldog</name>
    <name>puppy</name>
  </a>
  <a genName="horse">
    <name>horse</name>
    <name>zebra</name>
    <name>pony</name>
  </a>
  <a genName="cat">
    <name>kittie</name>
    <name>kitten</name>
  </a>
 </my:Dict>

 <xsl:variable name="vDict" select=
  "document('')/*/my:Dict/a"/>

 <xsl:template match="/">
  <root>
   <xsl:variable name="vhungryCages">
    <xsl:apply-templates select=
    "/*/animalsDictionary/cage"/>
   </xsl:variable>

   <xsl:choose>
    <xsl:when test="$vhungryCages/*">
     <hungryAnimals>
       <xsl:copy-of select="$vhungryCages"/>
     </hungryAnimals>
    </xsl:when>
    <xsl:otherwise>
     <everythingIsFine/>
    </xsl:otherwise>
   </xsl:choose>
  </root>
 </xsl:template>

 <xsl:template match="cage">
  <xsl:if test="
  /*/shortOfSupply/*[my:isA(current()/@animal,@animal)]">

  <cage name="{@name}"/>
  </xsl:if>
 </xsl:template>

 <xsl:function name="my:isA" as="xs:boolean">
  <xsl:param name="pSpecName" as="xs:string"/>
  <xsl:param name="pGenName" as="xs:string"/>

  <xsl:sequence select=
   "$pSpecName = $vDict[@genName = $pGenName]/name"/>
 </xsl:function>
</xsl:stylesheet>

When this transformation is applied on the provided XML document (corrected to be well-formed):

<root>
    <shortOfSupply>
        <food animal="doggie"/>
        <food animal="horse"/>
    </shortOfSupply>
    <animalsDictionary>
        <cage name="A" animal="kittie"/>
        <cage name="B" animal="dogs"/>
        <cage name="C" animal="cow"/>
        <cage name="D" animal="zebras"/>
    </animalsDictionary>
</root>

the wanted, correct result is produced:

<root>
   <hungryAnimals>
      <cage name="B"/>
      <cage name="D"/>
   </hungryAnimals>
</root>

Explanation: Do note the use of the XSLT current() function.

Answer 2

XPath 1.0 is not "relationally complete" - it can't do arbitrary joins. If you're in XSLT, you can always get round the limitations by binding variables to intermediate nodesets, or (sometimes) by using the current() function.

XPath 2.0 introduces range variables, which makes it relationally complete, so this limitation has gone.

Answer 3

Doesn't <xsl:when test="cage[@animal = /root/shortOfSupply/food/@animal]"> suffice to express your test condition?

Answer 4

Notice The dot operator in XPath is related to the current context. In XSLT the current template context_ is given by the function current(), which most of the time (not always) coincides with the ..

---

You can perform the test (and the apply templates as well), using the parent axis abbreviation (../):

 cage[@animal=../../shortOfSupply/food/@animal]

Moreover the match pattern in the the first template is wrong, it should be relative to the root:

 /root/animalsDictionary

@Martin suggestion is also obviously correct.

Your final template slightly modified:

<xsl:stylesheet 
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    version="1.0">

    <xsl:output method="xml" indent="yes" omit-xml-declaration="yes"/>
    <xsl:strip-space elements="*"/>

    <xsl:template match="root/animalsDictionary">
        <xsl:choose>
            <xsl:when test="cage[@animal=../../shortOfSupply/food/@animal]">
                <hungryAnimals>
                    <xsl:apply-templates select="cage[@animal
                            =../../shortOfSupply/food/@animal]"/>
                </hungryAnimals>
            </xsl:when>
            <xsl:otherwise>
                <everythingIsFine/>
            </xsl:otherwise>
        </xsl:choose>
    </xsl:template>

    <xsl:template match="cage">
        <cage name="{@name}"/>
    </xsl:template> 

</xsl:stylesheet>