I will post here the answer to your support ticket on the same topic, in case someone else hits the same problem:
It is not possible to extend the read-only navigation property to become read-write.
What you can do is to define a partial extension class for the PO10000 entity type and define there a new read-only property that filters the PO10001 items on demand:
.PO10001.Where(p => p.ARCHV_FLAG ==
)).OrderBy(p => p.PO10001_ID); }
Putting this code in a partial class in another code file will protect it from being overwritten when your data model changes. Also this implementation will guarantee that you always get the latest data from the database on each call of the FilteredPO10001 property.
the Telerik team
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