Adding custom fields to a Solr schema to enable sorting and grouping

example.xml

<!-- typical filesystem location ../config/spring/api/discovery.xml -->
<!-- sort filters for the discovery search-->
<property name="searchSortConfiguration">
    <bean class="org.dspace.discovery.configuration.DiscoverySortConfiguration">
        <property name="defaultSortOrder" value="desc"/>
        <property name="sortFields">
            <list>
                <ref bean="sortTitle" />
                <ref bean="sortDateIssued" />
                <ref bean="sortSomeField" />
            </list>
        </property>
    </bean>
</property>

<!-- followed by the definitions of the properties later in the saem file -->

<!--Sort properties-->
<bean id="sortTitle" class="org.dspace.discovery.configuration.DiscoverySortFieldConfiguration">
    <!-- definition below defaults to sort by alpha -->
    <property name="metadataField" value="dc.title"/>
</bean>

<bean id="sortDateIssued" class="org.dspace.discovery.configuration.DiscoverySortFieldConfiguration">
    <!-- in this case we want to sort by date, so that is specified explicitly -->
    <property name="metadataField" value="dc.date.issued"/>
    <property name="type" value="date"/>
</bean>

<bean id="sortSomeField" class="org.dspace.discovery.configuration.DiscoverySortFieldConfiguration">
    <!-- here we explicitly type the field as an integer -->
    <property name="metadataField" value="dc.some.field"/>
    <property name="type" value="integer"/>
</bean>