How to filter node list based on the contents of another node list

I'd like to use XSLT to filter a node list based on the contents of another node list. Specifically, I'd like to filter a node list such that elements with identical id attributes are eliminated from the resulting node list. Priority should be given to one of the two node lists.

The way I originally imagined implementing this was to do something like this:

<xsl:variable name="filteredList1" select="$list1[not($list2[@id_from_list1 = @id_from_list2])]"/>

The problem is that the context node changes in the predicate for $list2, so I don't have access to attribute @id_from_list1. Due to these scoping constraints, it's not clear to me how I would be able to refer 开发者_高级运维to an attribute from the outer node list using nested predicates in this fashion.

To get around the issue of the context node, I've tried to create a solution involving a for-each loop, like the following:

    <xsl:variable name="filteredList1"> 
        <xsl:for-each select="$list1">
            <xsl:variable name="id_from_list1" select="@id_from_list1"/>

            <xsl:if test="not($list2[@id_from_list2 = $id_from_list1])">
                <xsl:copy-of select="."/>
            </xsl:if>
        </xsl:for-each>
    </xsl:variable>

But this doesn't work correctly. It's also not clear to me how it fails... Using the above technique, filteredList1 has a length of 1, but appears to be empty. It's strange behaviour, and anyhow, I feel there must be a more elegant approach.

I'd appreciate any guidance anyone can offer. Thanks.

Use this XPath one-liner:

$vList1[not(@id = $vList2/@id)]

As far as I am aware using $var[] syntax doesn't work. What works is: expr1/[expr2 = $var], and func1($var).

What you can do is simply embed the expression that yields $list2 in the if test:

<xsl:for-each select="$list1">
  <xsl:variable name="id" select="@id_from_list1"/>
  <xsl:if test="not(expr2[@id_from_list2 = $id ])">
    <xsl:copy-of select="."/>
  </xsl:if>
</xsl:for-each>
<xsl:copy-of select="$list2"/>

Substitute expr2 with actual expression.