XSLT Concat fields together

Question

i am trying to filter on a specific field and concat on another field:

Input:

<?xml version="1.0" encoding="UTF-8"?>
<payloads>
    <payload>
        <firstname>michael</firstname>
        <secondname>brown</secondname>
        <number>1</number>
    </payload>
    <payload>
        <firstname>michael</firstname>
        <secondname>brown</secondname>
        <number>2</number>
    </payload>
    <payload>
        <firstname>michael</firstname>
        <secondname>brown</secondname>
        <number>3</number>
    </payload>
</payloads>

Output:

<?xml version="1.0" encoding="UTF-8"?>
<payloads>
    <payload>
        <firstname>michael</firstname>
        <secondname>brown</secondname>
        <number>1,2,3</number>
    </payload>
</payloads>

I know that i need to loop through each payload tag, but at the moment i am unable to get it too output correctly. At the moment i have this:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
    <xsl:output indent="yes"/>

    <xsl:template match="@* | node()">
        <xsl:copy>
            <xsl:apply-templates select="@* | node()"/>
        </xsl:copy>
    </xsl:template>

    <xsl:template match="payloads">
        <xsl:copy>
            <xsl:for-each select="payload">
                <payload>
                    <xsl:value-of select="firstname"/>
                    <xsl:value-of select="secondname"/>
                    <xsl:value-of select="number"/>
                </payload>
            </xsl:for-each>
        </xsl:copy>
    </xsl:template>

</xsl:stylesheet>

Answer 1

Use this template:

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
    <xsl:output method="xml" indent="yes"/>
    <xsl:strip-space elements="*"/>
    <xsl:key name="k" match="payload" use="concat(firstname, '|', secondname)"/>

    <xsl:template match="payload[generate-id() = 
                  generate-id(key('k', concat(firstname, '|', secondname)))]">
        <xsl:copy>
            <xsl:copy-of select="firstname"/>
            <xsl:copy-of select="secondname"/>
            <number>
                <xsl:for-each select="key('k', concat(firstname, '|', secondname))">
                    <xsl:value-of select="number"/>

                    <xsl:if test="position() != last()">
                        <xsl:text>,</xsl:text>
                    </xsl:if>
                </xsl:for-each>
            </number>
        </xsl:copy>
    </xsl:template>

    <xsl:template match="payload"/>

    <xsl:template match="@* | node()">
        <xsl:copy>
            <xsl:apply-templates select="@* | node()"/>
        </xsl:copy>
    </xsl:template>
</xsl:stylesheet>

When applied to provided input XML, it outputs wanted correct result:

<payloads>
  <payload>
    <firstname>michael</firstname>
    <secondname>brown</secondname>
    <number>1,2,3</number>
  </payload>
</payloads>