Why am I unable to sum up javabean subitem's value in jrtable?

Question

I get this exception when tying to sum nested column values

net.sf.jasperreports.engine.design.JRValidationException: Report design not valid : 
 1. Field not found : episodeCount
 2. Variable not found : episodeTotal

How would I pass the above parameter/variable(s) to the sub dataset?

My jrxml file

<subDataset name="dataset1">
    <field name="orderItem" class="com.blahblah.OrderItemPDF">
        <fieldDescription><![CDATA[_THIS]]></fieldDescription>
    </field>
</subDataset>

<field name="orderItems" class="java.util.Collection"/>
<field name="episodeCount" class="java.lang.Integer">
    <fieldDescription><![CDATA[$F{orderItem}.getEpisodeCount()]]></fieldDescription>
</field>

<variable name="episodeTotal" class="java.lang.Integer" calculation="Sum">
    <variableExpression><![CDATA[$F{episodeCount}]]></variableExpression>
</variable>

<title>
    <band height="113" splitType="Stretch">

    </band>
</title>
<pageHeader>
    <band height="360">
        <componentElement>
            <reportElement x="67" y="181" width="360" height="100"/>
            <jr:table xmlns:jr="http://jasperreports.sourceforge.net/jasperreports/components"
                      xsi:schemaLocation="http://jasperreports.sourceforge.net/jasperreports/components http://jasperreports.sourceforge.net/xsd/components.xsd">
                <datasetRun subDataset="dataset1">
                    <dataSourceExpression>
                        <![CDATA[new net.sf.jasperreports.engine.data.JRBeanCollectionDataSource($F{orderItems})]]></dataSourceExpression>
                </datasetRun>
                <jr:column width="130">
                    <jr:tableHeader height="30">
                        <staticText>
                            <reportElement x="0" y="0" width="130" height="20"/>
                            <textElement/>
                            <text><![CDATA[Media Families]]></text>
                        </staticText>
                    </jr:tableHeader>
                    <jr:detailCell height="20">
                        <textField>
                            <reportElement x="0" y="0" width="130" height="20"/>
                            <textElement/>
                            <textFieldExpression>
                                <![CDATA[$F{orderItem}.getMediaFamilyName()]]></textFieldExpression>
                        </textField>
                    </jr:detailCell>
                </jr:column>
                <jr:column width="130">
                    <jr:tableHeader height="30">
                        <staticText>
                        <reportElement x="0" y="0" width="130" height="20"/>
                        <textElement/>
                        <text><![CDATA[Episodes]]></text>
                        </staticText>
                    </jr:tableHeader>
                    <jr:detailCell height="20">
                        <textField>
                            <reportElement x="0" y="0" width="130" height="20"/>
                            <textElement/>
                            <textFieldExpression><![CDATA[$F{orderItem}.getEpisodeCount()]]></textFieldExpression>
                        </textField>
                    </jr:detailCell>
                </jr:column>
                <jr:column width="60">
                    <jr:tableHeader height="30">
                        <staticText>
                            <reportElement x="0" y="0" width="60" height="20"/>
                            <textElement/>
                            <text><![CDATA[AdUnits]]></text>
                        </staticText>
                    </jr:tableHeader>
                    <!--<jr:columnFooter height="20">-->
                        <!--<textField>-->
                            <!--<reportElement x="0" y="0" width="130" height="20"/>-->
                            <!--<textElement/>-->
                            <!--<textFieldExpression><![CDATA[$V{episodeTotal}]]></textFieldExpression>-->
                        <!--</textField>-->
                    <!--</jr:columnFooter>-->
                    <jr:detailCell height="20">
                        <textField>
                            <reportElement x="0" y="0" width="60" height="20"/>
                            <textElement/>
                            <textFieldExpression><![CDATA[$F{orderItem}.getAdUnitCount()]]></textFieldExpression>
                        </textField>
                    </jr:detailCell>
                </jr:column>
            </jr:table>
        </componentElement>
    </band>
</pageHeader>
<columnHeader>jas
</columnHeader>
<detail>
    <band/>
</detail>

Answer 1

When using a subDataset you need to define all your fields, parameter and variables inside of the subDataset. You can not reference fields, parameters and variables that are outside.

In your example the definition of the variable inside the subDataset would be something like this

<subDataset name="dataset1">
    <field name="orderItem" class="com.blahblah.OrderItemPDF">
        <fieldDescription><![CDATA[_THIS]]></fieldDescription>
    </field>
    <variable name="episodeTotal" class="java.lang.Integer" calculation="Sum">
        <variableExpression><![CDATA[$F{orderItem}.getEpisodeCount()]]></variableExpression>
    </variable>
</subDataset>