XPath - Selecting nodes with one similar and one different attribute - xslt

Question

So I'm having trouble understanding why one XPath expression gets the nodes I want, while the other doesn't.
First, the xml:
<doc>
<source id="225" clientID="567" matterID="225" level="2" />
<source id="226" clientID="993" matterID="226" level="2" />
<dest id="185" level="7" />
<dest id="226" level="7" />
</doc>
The keys in my xsl template are defined as follows:
<xsl:key name="sourceId" match="//source" use="#id" />
<xsl:key name="destId" match="//dest" use="#id" />
<xsl:key name="destLevel" match="//dest" use="#level" />
What I'm looking for are the source nodes, that match dest nodes on id, but have a different level attribute. The apply template that I figured would work in my head is the following:
<xsl:apply-templates select="source[key('destId', #id) and not(key('destLevel', #level))]" mode="update" />
But that doesn't seem to work. A colleague suggested putting a not around an expression that matches the nodes I don't want, and after a lot of trial and error, I thought this might work, to no effect:
<xsl:apply-templates select="source[not(not(key('destId', #id)) or not(key('destLevel', #level)))]" mode="update" />
Can anyone please walk me through what I need in order to solve this?
Edit: I previously thought I'd solved this with the second query, but it seems I was mistaken.
====Solution====
Dimitre Novatchev has a detailed breakdown of different ways to solve this, but my ultimate solution was actually slightly different than his.
In essence, I created a virtual key with the concat() function that combined the two attributes. That way, I could find nodes that matched the id, but not the id-level combo.
Extra key:
<xsl:key name="destByIdAndLevel" match="//dest" use="concat(#id,'+',#level)" />
Changed apply-template call:
<xsl:apply-templates select="source[key('destId', #id) and not(key('destByIdAndLevel',concat(#id,'+',#level)))]" mode="update" />

Answer 1

I. This transformation:
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output omit-xml-declaration="yes" indent="yes"/>
<xsl:strip-space elements="*"/>
<xsl:template match="source">
<xsl:copy-of select=
"self::*[../dest[#id = current()/#id and not(#level=current()/#level)]]"/>
</xsl:template>
</xsl:stylesheet>
when applied on this XML document (an additional source element added to the provided XML document -- to verify more cases):
<doc>
<source id="185" clientID="567" matterID="225" level="7" />
<source id="225" clientID="567" matterID="225" level="2" />
<source id="226" clientID="993" matterID="226" level="2" />
<dest id="185" level="7" />
<dest id="226" level="7" />
</doc>
produces the wanted, correct result:
<source id="226" clientID="993" matterID="226" level="2"/>
II. Solution using one key:
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output omit-xml-declaration="yes" indent="yes"/>
<xsl:key name="kDestById" match="dest" use="#id"/>
<xsl:template match="/*">
<xsl:copy-of select=
"source[key('kDestById',#id)
and
not(#level=key('kDestById',#id)/#level)]"/>
</xsl:template>
</xsl:stylesheet>
When this transformation is applied on the same XML document (above), again the same wanted, correct result is produced:
<source id="226" clientID="993" matterID="226" level="2"/>
III. Solution with two keys:
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output omit-xml-declaration="yes" indent="yes"/>
<xsl:key name="kDestById" match="dest" use="#id"/>
<xsl:key name="kDestByLevel" match="dest" use="#level"/>
<xsl:template match="source">
<xsl:copy-of select=
"self::*
[key('kDestById',#id)
and
key('kDestById',#id)
[not(count(.|key('kDestByLevel',current()/#level))
=
count(key('kDestByLevel',current()/#level))
)
]
]"/>
</xsl:template>
</xsl:stylesheet>
This again produces the wanted, correct result:
<source id="226" clientID="993" matterID="226" level="2"/>

Answer 2

source[key('destId', #id) and not(key('destLevel', #level))]
This gives source nodes which have an id the same as some dest nodes, and also have a level which is not the same as any dest nodes. Note that it doesn't require a particular dest node to have the same id but different level.
I think you are looking for something like this:
source[set:difference(key('destId', #id), key('destLevel', #level))]
set:difference() is from EXSLT.