Asp .NET Web API using MVVM pattern - asp.net

Question

I'm trying to understand how I should use the MVVM pattern for a CRUD operation. Currently I have methods in my API controller such as below. My question is: using MVVM pattern, should I still build my api like that (e.g., accessing DB)? Or should it be changed? If nothing changes, in which case I would implement ViewModels and how they should be managed by the API? I made some researches but it's still not clear for me.
public IHttpActionResult GetProduct(int id)
{
var product = _context.Products.SingleOrDefault(p => p.Id == id);
return Ok(product);
}
[HttpPost]
public IHttpActionResult CreateProduct(Product product)
{
...
_context.Products.Add(product);
_context.SaveChanges();
return Created(new Uri(Request.RequestUri + "/" + product.Id), product);
}

Answer 1

I think part of the problem is that you don't understand the role of MVVM in a web application. To understand, you have to look at a web application as composed of two separate applications--the server side, and the client side.
Server side, the pattern being used is MVC (not surprising, it's called ASP.NET MVC for a reason). If you're trying to fit your understanding of MVVM around the MVC pattern--don't. It doesn't apply. The MVC pattern on the server is simple to understand and implement; don't try to crowbar any MVVM into it. Just design your server side code using the server side pattern.
Client side is a different matter. By default, ASP.NET MVC using Razor pages renders your HTML on the server and delivers it for display to the client. And, typically, that HTML is designed to respond to users interaction with the page by posting back to the server. Your controller methods interpret these postbacks, perform the required logic, and render the correct razor page in response. But this isn't the only way to write a website.
More and more common, people are designing Single Page Applications. These use ajax callbacks to send the result of user actions back to the server, and then process the response. Server side, these requests are almost always handled through a WebApi controller method, not an ASP.NET MVC controller method. The request comes across the wire as ajax, and the result is usually returned as ajax. No HTML gets rendered. This is where MVVM comes into play.
Client side, these web pages typically use a javascript MVVM (ish) framework, like knockoutjs or angular. The response json is used to update view models that are bound to HTML elements in the web page. These frameworks handle synchronizing updates and user actions between the UI and these view models, just like Bindings synchronize updates between the UI and your view models in WPF.
Simply put, MVC is its own thing, and MVVM usually only exists within the client side of a website, and usually only when the website extensively uses ajax callbacks instead of postbacks.
And, to answer your direct question, if you're using MVVM on the client side, using ajax calls to perform your CRUD ops, you should design your WebApi controller methods to Post/Get/Put/Delete your data.

Answer 2

Personally, I always use the Repository Pattern for anything to do with CRUD operations, such as interacting with an entity in a database.
I would create a separate class called "ProductRepository" and put all the methods to Get, Create, Update, and Delete a Product in that class. This way, only your Respository cares about the details of interacting with the database and performing the CRUD operations on it. Your entire application should not care about how this gets done, only your Repository class. This is something known as the Single Reponsibility Principle. It is part of the "SOLID" principles of design and architecture.
Then, in either your Controller or ViewModel or where ever you need it to happen, you just have to instantiate the ProductRepository and use its methods throughout your application.
If you use a separate Web API for your service layer and data access layer, then you don't really need MVC. You just need a front-end framework to consume the Web API URLs, like AngularJS or any other JS framework of your choice.
If you want to use MVC, then you don't really need Web API. You can just build your service layer and data access layer into the MVC application, or build them as separate projects (class libraries) and include them in your overall project solution. It all just depends upon how you want the architecture to be and how complex you want to go with it.
But either way, a "ProductRepository" should be involved -- whether it is in your Web API (if you go that route), or in your MVC project (if you go that route). The ultimate goal is to have separation of concerns. You want to separate your business logic layer from your data access layer. You don't want to be making direct calls to the database throughout your entire application. That will result in code that is very tightly coupled and hard to test and maintain over time. If you ever change out databases in the future, you only want to update one place in the code, instead of many places.
Hope this helps you out some! Best regards and happy coding!!!