如何组成视图模型并同时最小化数据库交互

Question

In Asp.Net MVC with Entity Framework, I totally agree that repositories in the DAL (Data Access Layer), which is located in a separate assembly, must not deal with view models.

From the DAL I get entities, mostly IQueryAbles. From those I compose quite complex view models. The challenge is that I want to minimize the number of sql selects generated in order to compose a view model. In some cases, I need to query the db 10-20 times.

for example, this is my view model

class ViewModel{
    //.. public properties mapped from entities   1 sql select
    public int CountOfSomething {get; set;} 1 sql select
    public int CountOfSomethingElse {get;set;} 1 sql select
    //..and so on
}

So this is not optimal, from a performance perspective. I could modify the repositories to return much more complex results, but then they would not return entities, but something else.

In that case: DAL composes and returns an intermediate model (entity(s) plus additional data) which is defined in the DAL assembly, and THAT intermediate model, and not an entity, is then translated into a view model.

DB -> Entity -> Intermediate model -> ViewModel

This additional "mapping" requires more code == more complexity. I am not sure how to tackle this problem now.

Until now I have solved some cases with creating additional "helper" repository methods, but I realize it very rapidly becomes a bit of a mess.

Any ideas?

Answer 1

Make some classes which job is only to map from the entity/intermediate model to the viewmodel.

There are some libraries out there for that purpose, I recommend you automapper , it even has some testing functionalities so you can do your mappings properly.