changing context node

hello all.
I have a problem which  is as follows..

I am looping through an external document as in..
<xsl:variable name="external-merge-xml"
select="document('merge-spec.xml')"/>
  <xsl:for-each select="$external-merge-xml/Merging-Data/DBDetails">
<xsl:variable name = "dbDetails-id" select = "@id"/>
<!-- here i want to go through the source xml and match an id which is equal
to the $dbDetails-id.-->
<!--so-->
<!-- do either -->
<xsl:for-each select = "MainSrcDB/dbDetails[@id = $dbDetails-id]">
  -- some processing--
</xsl:for-each>
<!--or-->
<xsl:for-each select = "MainSrcDB/dbDetails>
<xsl:if test = "@id = $dbDetails-id">
    -- some processing--
</xsl:if>
</xsl:for-each>
<!--or -->
<xsl:apply-templates match = "/MainSrcDB/dbDetails"/>
 <!-- and define a template that matched dbDetails element -->
</xsl:for-each>

I am not able to do this because the context node is
$external-merge-xml/Merging-Data/DBDetails..
and the for-each doesnt produce any result..

Is there any other way to do this?
Thanks


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