MVC + 3 tier; where ViewModels come into play?
- by mikhairu
I'm designing a 3-tiered application using ASP.NET MVC 4. I used the following resources as a reference.
CodeProject: MVC + N-tier + Entity Framework
Separating data access in ASP.NET MVC
I have the following desingn so far.
Presentation Layer (PL) (main MVC project, where M of MVC was moved to Data Access Layer):
MyProjectName.Main
Views/
Controllers/
...
Business Logic Layer (BLL):
MyProjectName.BLL
ViewModels/
ProjectServices/
...
Data Access Layer (DAL):
MyProjectName.DAL
Models/
Repositories.EF/
Repositories.Dapper/
...
Now, PL references BLL and BLL references DAL. This way lower layer does not depend on the one above it.
In this design PL invokes a service of the BLL. PL can pass a View Model to BLL and BLL can pass a View Model back to PL.
Also, BLL invokes DAL layer and DAL layer can return a Model back to BLL. BLL can in turn build a View Model and return it to PL.
Up to now this pattern was working for me. However, I've ran into a problem where some of my ViewModels require joins on several entities. In the plain MVC approach, in the controller I used a LINQ query to do joins and then select new MyViewModel(){ ... }. But now, in the DAL I do not have access to where ViewModels are defined (in the BLL).
This means I cannot do joins in DAL and return it to BLL. It seems I have to do separate queries in DAL (instead of joins in one query) and BLL would then use the result of these to build a ViewModel. This is very inconvenient, but I don't think I should be exposing DAL to ViewModels.
Any ideas how I can solve this dilemma? Thanks.
