Domain Model, Data Mapper, Repository, Unit of Work Patterns and Lazy Load, oh my!

Our efforts to create a library for building enterprise-level LOB has led us to the Domain Model, Data Mapper, Repository, Unit of Work and Lazy Load patterns, just like everybody else. I just got my copy of Nilsson’s Applying Domain-Driven Design and Patterns and enjoy it very much. I believe Rob Conery commented on the pattern of patterns approach that defines Domain-Driven Design. That statement, while a bit incomplete, appears to hold true.

Aside from well-documented pitfalls, domain-driven design appears quite fresh and appealing, at least to me. Nevertheless, I ran into several hurdles while trying to implement the Unit of Work and Repository patterns. In particular, where do I add the Data Mapper? I’ve puzzled over this the last few days without any good answers.

This evening as we were walking out the door, JB and I decided we’d finally look into Expressions and building our own IQueryable implementation so that we could lazy load our mapped domain models. Lo and behold, I think this answers my other questions, as well, as building the query expression also allows us to map to the data provider, no matter what that may be, including Entity Framework or LINQ to SQL Expression trees.

Implementing IQueryable also allows us to catch each object as it is pulled through the iterator and tag it for change tracking. Now many reading this post will probably be thinking that this sounds like a lot of work—and I’m sure it is—and that these benefits were supposed to be delivered through Entity Framework, LINQ to SQL, etc. Well, you are right, but due to the lack of interfaces and current limitations, these may not always be the best options.

I’ve just started learning this myself, so I can only point you to the how, not the how hard. Check out the LINQ: Building an IQueryable Provider Series. As I progress, you can be I’ll be commenting on the ease or difficulty of building a custom IQueryable provider. Several others appear to have gone this route, so it can’t be a completely worthless pursuit (see SubSonic and IQToolkit).

Anyone else taken the plunge? What were your experiences?

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