ASP.Net MVC - ModelState.AddModelError when GET/POST have different models

Question

I have a use case where I used different models for the GET and POST actions in my controller. This works great for my view, because most of the data goes into labels. The model for the GET method contains 10 properties, but the model for the POST method only needs 3.

This GET view renders a form, which only needs 3 of these properties, not all 10. Thus, the model for the POST method accepts a model class which contains only these 3 properties. Therefore, the ASP.Net MVC model binder populates the model class parameter on my POST method with only these 3 necessary properties, and all is well.

Here's the question: When I encounter some business rule violation in the POST method, and want to use ModelState.AddModelError, and re-display the original view, I no longer have the 7 properties that were not POSTed, as they were not part of the form, and are not part of the model class which this method takes as its parameter.

Currently, I'm calling into a builder to return an instance of the model class for the POST method, and have the GET method itself delegating to the same builder. So, in these cases, when there is some business rule violation in the POST method, I return a View("OriginalGetView", originalGetModel). How can I use ModelState.AddModelError in this case, in the POST method, if I want to send custom messages back to the view, using a completely different model class?

It seemed way too lazy to use the same model class for both the GET and POST methods, given that their needs were so different. What is the best practice here? I see a lot of people recommending to use the same model for both methods, and to POST all of the fields back from hidden form fields, but that just seems like a waste of bandwidth in the majority of cases, and it feels ugly to be sending things like "VendorName" back to the server, when I already have "VendorId".

Answer 1

I may be misunderstanding what you are trying to do, but make sure you aren't being penny-wise and pound foolish. I see you may only want to post the identifiers and not necessarily the descriptors. But it sounds like you have to re-display the view after posting...if so you can just access the model properties if you post the same model that is in the get. If you only post the identifiers, you have to spend time re-accessing the database to get the description values(i.e. vendorname as you describe) using the vendor id no? Wouldn't that also be extra processing? Like I said, I could be misunderstanding your post, but for consistency using the same view model for your view to get and post makes the most sense to me.