Experiences with LINQ to SQL O/R mapper

 I have been experimenting with LINQ to SQL. It seems like a nice tool for accessing existing databases and for database first or database driven development. For object driven approach this tool has some rather unpleasant limitations. So far I have encountered following “problems”:

1. LINQ to SQL supports only “one table per inheritance hierarchy” – persisting strategy. Creating a base class with common attributes like Id, timestamps or owners for all persistent objects does not sound like a good idea any more.
2. LINQ to SQL does not seem to support many to many relations transparently. If your model has many to many relations you have to introduce artificial relation class to your object model having primary keys of related entities as properties.
3. It seems that you can not have two automatically updating DateTime properties in your classes (Type = DateTime, Time Stamp = true). I was trying to auto create a table that has properties “CreatedAt” and “ModifiedAt” and make the first one synchronize on insert and the other one on updates. Instead of a auto generated table I got error “System.InvalidOperationException: Members ‘System.DateTime CreatedAt’ and ‘System.DateTime ModifiedAt’ both marked as row version.”.
4. I don’t really know where I should define the default values for my persistent classes. Maybe by writing a partial method to OnCreated()?

Luckily we still have tools like Castle Active Record (or NHibernate) for object driven development. And you might be able to use LINQ with NHibernate.