I am struggling to find a way to make POCOs work with Linq-to-Sql when my domain model is not table-driven - meaning that my domain objects do not match-up with the database schema.
For example, in my domain layer I have an Appointment object which has a Recurrence property of type Recurrence. This is a base class with several subclasses each based on a specific recurrence pattern.
In my database, it makes no sense to have a separate AppointmentRecurrences table when there is always a one-to-one relationship between the Appointment record and its recurrence. So, the Appointments table has RecurrenceType and RecurrenceValue columns. RecurrenceType has a foreign key relationship to the RecurrenceTypes table because there is a one-to-many relationship between the recurrence type (pattern) and the Appointments table.
Unless there is a way to create the proper mapping between these two models in Linq-to-Sql, I am left with manually resolving the impedence mismatch in code.
This becomes even more difficult when it comes to querying the database using the Specification pattern. For example, if I want to return a list of current appointments, I can easily create a Specification object that uses the following Expression: appt => appt.Recurrence.IsDue. However, this does not translate into the Linq-to-SQL space because the source type of the Expression is not one that L2S recognizes (e.g. it's not the L2S entity).
So how can I create the complex mapping in Linq-to-SQL to support my domain model?
Or, is there a better way to implement the Specification pattern in this case? I'd thought about using interfaces that would be implemented by both my domain object and the L2S entity (through partials) but that's not possible with the impedence mismatch of the two object graphs.
Suggestions?
Unfortunately, Linq to SQL pretty much forces you into a class-per-table model, it does not support mapping a single entity class to several database tables.
Even more unfortunately, there are very few ORM's that will support more complicated mappings, and vanishingly few that do and offer decent LINQ support. The only I'm even remotely sure of is NHibernate (our experiences with Entity Framework rate it really no better than L2S in this regard).
Also, trying to use the specification pattern in LINQ expressions is going to be quite the challenge.
Even with ORM's, and even with a really strong abstracting ORM like NHibernate, there is still a large impedence mismatch to overcome.