the persistent type
While the persistent objects are great places to store the information
we want to persist, we also need a place to keep track of “meta
information” about them. For instance, we want to be able to specify
keys, or properties of the object that uniquely identify a
persistent within a collection of persistents. We store this kind of
meta information in our PTypes.
It’s important to note that, while “key” and “index” are terms used in
database schemas and physical modelling, that is not our intention
here. Within our PTypes, we are still describing the domain, not the
mapping of the domain into database schema. Of course, longevity will
translate these things into database indexes and
keys, and it does so in a consistent and predictable way, but that
does not mean that the uniqueness of a set of properties is not part
of the domain. The key question to ask ourselves in making such a
determination is, “would a domain expert care about such things”?
They would clearly care that an account number is unique, but they
should not care about how that maps into a database schema.
Because we build our keys and indexes up out of properties, we will discuss properties first.