XSLT finding matching node by comparing element text to substring result - string

Question

I am processing this XML:
<Brand>
<Brand_Name>BLENDERM</Brand_Name>
<Brand_Code>1103</Brand_Code>
<Groups>
<Group>
<Group_Code>657</Group_Code>
<Parent_Code>0</Parent_Code>
<Group_Level>1</Group_Level>
<Group_Name>Brand Default</Group_Name>
<Product>
<Pip_code>0032359</Pip_code>
<Status>In Use</Status>
Using this XSLT:
<xsl:template match="Product" mode="phase-3">
<xsl:value-of select="document('rx_catmapping.xml')/descendant::mapping[source=substring(ancestor::Brand/Brand_Name,1,1)]/target"/>
</xsl:template>
Here is a sample of rx_catmapping.xml:
<Lookup xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<mapping>
<source>a</source>
<target>788</target>
</mapping>
<mapping>
<source>B</source>
<target>789</target>
</mapping>
</Lookup>
So, I am processing the Product element, which is a descendant of Brand. The first letter of Brand/Brand_Name in this case is B, and I am trying to output the value 789 by looking it up in rx_catmapping.xml. This should be really simple but I am completely stumped! I have tried changing the first part of the XPath to refer to document('rx_catmapping.xml')/Lookup/mapping, or document('rx_catmapping.xml')//mapping. I have also tried changing the first half of the comparison to string(source), or to source/text(), but neither of these works either. (The reason for trying this was that using source='B', for example, did seem to work, so I wondered if I was trying to compare two incompatible data types.)
Thanks in advance for your help.

Answer 1

Define a key
<xsl:key name="k1" match="mapping" use="source"/>
then use
<xsl:variable name="map-doc" select="document('rx_catmapping.xml')"/>
and
<xsl:variable name="letter" select="substring(ancestor::Brand/Brand_Name,1,1)"/>
<xsl:for-each select="$map-doc">
<xsl:value-of select="key('k1', $letter)/target"/>
</xsl:for-each>
With XSLT 2.0 you can simplify that to
<xsl:value-of select="key('k1', substring(ancestor::Brand/Brand_Name,1,1), $map-doc)"/>

Answer 2

The problem is, I believe, that at the point where you do ancestor::Brand/Brand_Name, the context is the mapping element in the external file. This worked for me
<xsl:template match="/">
<xsl:apply-templates select=".//Product"/>
</xsl:template>
<xsl:template match="Product">
<xsl:variable name="x" select="substring(ancestor::Brand/Brand_Name,1,1)"/>
<xsl:value-of select="document('file:///c:/temp/rx_catmapping.xml')//mapping[source=$x]/target"/>
</xsl:template>