Wouldn't creating ViewModels lead to redundancy? In the sense I have my domain model and I need to display the data from it on a view. So we create ViewModels, add DataAnnotations to it and display it on the View. At this point I have 2 object with almost identical data.
4 Answers
2
votes
As others have already said, only the most trivial application can get away with passing their domain models directly to the view. Even then, it's still not a good idea for a lot of reasons.
The requirements of your view are different from the requirements for your data model. For instance, you may have a field that is required in your view, but is nullable in your vie model. If you add a `[Required]' attribute, then your model will now consider this field non-nullable.
However, my single biggest reason for never using domain models in views is for security. MVC allows you to post any value to it, and the default model binder will happily plug values you post into the model, which means if you had an IsAdmin flag, and someone posted a true value for IsAdmin, then when you saved the changes to the model, someone is now an admin.
The first rule of web security is never trust input from the user, and passing your view models directly to the view basically gives up sanitizing your data.
1
votes
Yes, it is a form of redundancy. But redundancy is often required to achieve other goals. In case of Models, having this separation of view models and domain models helps in achieving a decoupled setup between your view and data-store. And it is not often that ViewModels are exact copies of Domain.
Which means, either can change without having an impact on other. I can see cases where this would be valuable - data-type changes in table need not call for deployment of the web application.
So, in summary, while there is redundancy, it is a design choice on whether the system is complex enough to benefit from this redundancy.
0
votes
In 99% cases ViewModels don't lead to redundency.
The only 1% which comes to my mind is simple application with anemic domain models and pages, which shows nothing but a single model on a page. This is peculiar to content management pages.
In any other case:
1) your ViewModels will join information from multiple domain models
2) you'll have a logic specific to your domain in domain models
3) it's not a good idea to mix view-specific metainformation like DataAnnotations into your domain
0
votes
Nope, using ViewModel has its own purpose. Let's think about a situation when your view has two or more Entities from the Domain Model, without a ViewModel, how are you gonna organize data? The data needed for a view sometime is not exactly like domain model, it can has less or more information and sometime we have to pre-process data from domain before rendering view(e.g. format date time depends on client's culture).
Furthermore, ViewModel help de-couple the Web UI from the domain layer. Entities in Domain Model is not just about data representation(which is the only purpose of view model), they also have operations that mimics the business rule, you don't want expose too much domain knowledge to the UI layer who doesn't need to know.
