Grouping by column in Jasper

Question

Im new with Jasper and I'm struggling with simple grouping.

My dto which is passed:

@Getter
public class OrderPrintPreviewInputData {
private final List<OrderedTestInputData> tests;
....
}

OrderedTestInputData.class:

@Getter
public class OrderedTestInputData {
private final Long id;

@Setter
private String testType;

@Setter
private String materialType;
}

now my jrxml:

<jasperReport xmlns="http://jasperreports.sourceforge.net/jasperreports" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://jasperreports.sourceforge.net/jasperreports http://jasperreports.sourceforge.net/xsd/jasperreport.xsd" name="pzt-order-print-preview" language="groovy" pageWidth="595" pageHeight="842" columnWidth="556" leftMargin="20" rightMargin="19" topMargin="20" bottomMargin="20" uuid="406fbc92-1eae-45c7-93f1-f65ed1808b63">
<property name="ireport.zoom" value="1.610510000000001"/>
<property name="ireport.x" value="0"/>
<property name="ireport.y" value="626"/>
...
<subDataset name="orderedTests" uuid="448c9b60-118e-407a-b2c6-61db0f377e4e">
    <field name="tests.testType" class="java.lang.String"/>
    <field name="tests.materialType" class="java.lang.String"/>
</subDataset>
<parameter name="SUBREPORT_JASPER_FILE" class="java.lang.Object"/>
<field name="tests" class="java.util.List"/>
...
<detail>
    <frame>
        <reportElement positionType="Float" x="0" y="525" width="556" height="26" uuid="393eb4ce-1fa9-4e44-a560-a94ab2e2a04f">
            <property name="com.jaspersoft.studio.element.name" value="ExaminationFrame"/>
            <property name="com.jaspersoft.studio.unit.y" value="px"/>
        </reportElement>
        <subreport>
            <reportElement positionType="Float" x="-3" y="16" width="556" height="10" uuid="8666b227-be06-4552-8a0d-12015455ea4a">
                <property name="com.jaspersoft.studio.unit.y" value="px"/>
            </reportElement>
            <dataSourceExpression>
                <![CDATA[new net.sf.jasperreports.engine.data.JRBeanCollectionDataSource($F{tests})]]> 
            </dataSourceExpression>
            <subreportExpression>
                <![CDATA[$P{SUBREPORT_JASPER_FILE}]]>
            </subreportExpression>
        </subreport>
    </frame>
</detail>

The data is displayed that way:

row 1 - testType_1 | materialType_1
row 2 - testType_1 | materialType_2
row 3 - testType_2 | materialType_3
row 4 - testType_3 | materialType_4
row 5 - testType_3 | materialType_5

but now I need to display the results grouped by testType like below:

row 1 - testType_1 | materialType_1 + '\n' + materialType_2
row 2 - testType_2 | materialType_3
row 3 - testType_3 | materialType_4 + '\n' + materialType_5

I tried with such code but without any results:

<variable name="MaterialTypesAggregated" class="java.lang.String" resetType="Group" resetGroup="TestTypeGroup" calculation="System">
    <variableExpression><![CDATA[(($V{MaterialTypesAggregated} != null ? $V{MaterialTypesAggregated} + ", " : "") + $F{tests.materialType})]]></variableExpression>
    <initialValueExpression><![CDATA[""]]></initialValueExpression>
</variable>
<group name="TestTypeGroup">
    <groupExpression><![CDATA[$F{tests.testType}]]></groupExpression>
    <groupHeader>
        <band height="20">
            <textField>
                <reportElement x="0" y="0" width="200" height="20" uuid="c4efc3da-c43d-4100-ad8d-0a0d358bf386"/>
                <textFieldExpression><![CDATA[$F{tests.testType}]]></textFieldExpression>
            </textField>
        </band>
    </groupHeader>
    <groupFooter>
        <band height="20">
            <textField>
                <reportElement x="0" y="0" width="400" height="20" uuid="b121bb43-86a9-4b7d-8e03-c81ccd938466"/>
                <textFieldExpression><![CDATA["Material Types: " + $V{MaterialTypesAggregated}]]></textFieldExpression>
            </textField>
        </band>
    </groupFooter>
</group>

with this code above I'm getting the errors:

Caused by: net.sf.jasperreports.engine.design.JRValidationException: Report design not valid : 
 1. Field not found : tests.materialType
 2. Field not found : tests.testType
 3. Field not found : tests.testType
 4. Field not found : tests.testType
 5. Field not found : tests.materialType