18.104.22.168. Value Datasources
Value datasources enable execution of queries that return scalar values and aggregates. For example, you can load some aggregated statistics for customers:
select o.customer, sum(o.amount) from demo$Order o group by o.customer
Value datasources work with entities of a special type named
KeyValueEntity. This entity can contain an arbitrary number of attributes which are defined at runtime. So in the example above, the
KeyValueEntity instances will contain two attributes: the first of type
Customer and the second of type
Value datasource implementations extend other widely used collection datasource classes and implement a specific interface:
ValueDatasource. Below is a diagram showing the value datasource implementations and their base classes:
ValueDatasource interface declares the following methods:
addProperty()- as the datasource can return entities with any number of attributes, you have to specify what attributes are expected by using this method. It accepts a name of the attribute and its type in the form of Datatype or a Java class. In the latter case, the class should be either an entity class or a class supported by one the datatypes.
setIdName()is an optional method which allows you to define one of the attributes as an identifier attribute of the entity. It means that
KeyValueEntityinstances contained in this datasource will have identifiers obtained from the given attribute. Otherwise,
KeyValueEntityinstances get randomly generated UUIDs.
getMetaClass()returns a dynamic implementation of the
MetaClassinterface that represents the current schema of
KeyValueEntityinstances. It is defined by previous calls to
Value datasources can be used declaratively in a screen XML descriptor. There are three XML elements corresponding to the implementation classes:
XML definition of a value datasource must contain the
properties element that defines the attributes of
KeyValueEntity instances that will be contained in the datasource (see the
addProperty() method description above). The order of
property elements should conform to the order of result set columns returned by the query. For example, in the following definition the
customer attribute will get its value from
o.customer column and the
sum attribute from
<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> </dsContext>
Value datasources are designed only for reading data, because
KeyValueEntity is not persistent and cannot be saved by standard persistence mechanisms.
You can create value datasources either manually or in Studio in the Datasources tab of the Screen designer page.
The Properties editor allows you to create the datasource attributes of a certain datatype and/or a Java class.