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);
public IHttpActionResult CreateProduct(Product product)
return Created(new Uri(Request.RequestUri + "/" + product.Id), product);
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.
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.