Declarative derived properties

[mdiep]

It can be useful to keep derived properties to denormalize some aspect of the data model. e.g., a denormalized book count on an author that is calculated from the number of books. This can make queries quicker.

PersistDB should provide some way to declare derived properties. The store should keep the field updated when changes are made. This behaves essentially like an observed query, but the result is persisted in the store.