Where to convert business model to view model?

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Repositories should return domain models, not view models. As far as the mapping between the models and the view models is concerned, personally I use AutoMapper so I have a separate mapping layer but this layer is called from the controller.

Here’s how a typical GET controller action might look like:

public ActionResult Foo(int id)
{
    // the controller queries the repository to retrieve a domain model
    Bar domainModel = Repository.Get(id);

    // The controller converts the domain model to a view model
    // In this example I use AutoMapper, so the controller actually delegates
    // this mapping to AutoMapper but if you don't have a separate mapping layer
    // you could do the mapping here as well.
    BarViewModel viewModel = Mapper.Map<Bar, BarViewModel>(domainModel);

    // The controller passes a view model to the view
    return View(viewModel);
}

which of course could be shortened with a custom action filter to avoid the repetitive mapping logic:

[AutoMap(typeof(Bar), typeof(BarViewModel))]
public ActionResult Foo(int id)
{
    Bar domainModel = Repository.Get(id);
    return View(domainModel);
}

The AutoMap custom action filter subscribes to the OnActionExecuted event, intercepts the model passed to the view result, invokes the mapping layer (AutoMapper in my case) to convert it to a view model and substitutes it for the view. The view is of course strongly typed to the view model.

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