How to use subclasses (type cast)

If I understand you correct you expect something  like

INSERT INTO new.MESSAGE (SELECT * from  EnsLib.HL7.Message  where id = ? )

this may work for rather modest designed relational tables but I doubt it will work for some complex structure as EnsLib.HL7.Message .

Subclassing is not the way to solve your problem for 2 reasons:

• A subclass has always to match the .IsA() class type of its parent    (e.g. Employee as subclass of Person. Employee is also always a Person)
• SubClasses share the storage with its parent. There are dirty tricks to avoid this but I assume you do not intend to change  EnsLib.HL7.Message.

There are ways to do something like this if you fiddle around in the storage structures. 
BUT: for reasons of transparency, documentation, and maintainability
I'd strongly recommend to use DTL. 

Fiddling around with COS in Ensemble is a rather tricky exercise.
Based on practical experience I can confirm that your successors will never forget you.

Hi Abel,

When we subclass a persistent class and we need that this subclass have a own storage we need to add in the inheritance class list the %Persistent class  first of all classes.

Shared storage

Class sample.MyHL7 Extends EnsLib.HL7.Message
{ 

Storage Default
{
<Type>%Library.CacheStorage</Type>
}

}

Own storage

Class sample.MyHL7 Extends (%Persistent, EnsLib.HL7.Message)
{ 

Storage Default
{
<Data name="MyHL7DefaultData">
<Value name="1">
<Value>%%CLASSNAME</Value>
</Value>
<Value name="2">
<Value>ParentId</Value>
</Value>
<Value name="3">
<Value>DocType</Value>
</Value>
<Value name="4">
<Value>Envelope</Value>
</Value>
<Value name="5">
<Value>Source</Value>
</Value>
<Value name="6">
<Value>IsMutable</Value>
</Value>
<Value name="7">
<Value>OriginalDocId</Value>
</Value>
<Value name="8">
<Value>MessageTypeCategory</Value>
</Value>
<Value name="9">
<Value>TimeCreated</Value>
</Value>
</Data>
<Data name="UserValues">
<Attribute>UserValues</Attribute>
<Structure>subnode</Structure>
<Subscript>"UserValues"</Subscript>
</Data>
<DataLocation>^sample.MyHL7D</DataLocation>
<DefaultData>MyHL7DefaultData</DefaultData>
<IdLocation>^sample.MyHL7D</IdLocation>
<IndexLocation>^sample.MyHL7I</IndexLocation>
<StreamLocation>^sample.MyHL7S</StreamLocation>
<Type>%Library.CacheStorage</Type>
} 

}