Using ValueDataSource to show skalar value - cuba-platform

Question

I want to show a skalar values by using a label. The following example shows the sum of customer order amounts.
https://doc.cuba-platform.com/manual-6.8/value_datasources.html
<dsContext>
<valueCollectionDatasource id="salesDs">
<query>
<![CDATA[select o.customer, sum(o.amount) from demo$Order o group by o.customer]]>
</query>
<properties>
<property class="com.company.demo.entity.Customer" name="customer"/>
<property datatype="decimal" name="sum"/>
</properties>
</valueCollectionDatasource>
here I want to bind the loaded data to the label:
<label datasource="salesDs"
property="name"/>
but nothing is shown.
Why is the label value empty? (salesDs is correctly loaded, I can step through with intelliJ...)
I also tried to get the loaded data from the datasource, but I can't find the correct way.
salesDs.getItem() //returns null
salesDs.getItems() //retruns a collection of KeyValueEntries
But, what is the correct way to get my data from a KeyValueEntry?

Answer 1

It would work if you selected an item in the collection datasource (no matter ValueCollectionDatasource or a normal one). It can be done programmatically via setItem() or by some visual component also connected to the same datasource.
For example, in the below screen the label shows the name of the customer currently selected in the table:
<dsContext>
<valueCollectionDatasource id="customersDs">
<query>
<![CDATA[select e.name, e.email
from sales$Customer e]]>
</query>
<properties>
<property datatype="string"
name="name"/>
<property datatype="string"
name="email"/>
</properties>
</valueCollectionDatasource>
</dsContext>
<layout expand="customersTable" spacing="true">
<table id="customersTable" width="100%">
<columns>
<column id="name"/>
<column id="email"/>
</columns>
<rows datasource="customersDs"/>
</table>
<groupBox caption="Label">
<label id="nameLab"
datasource="customersDs"
property="name"/>
</groupBox>
</layout>