How to convert ModelState errors to valid JSON API response

[rhyek]

My API does some validation on POST/PUT requests on some non-JSONAPI endpoints and may generate an error response using ModelState. When trying to convert these endpoints to JSONAPI, I notice Saule isn't currently trying to use the information available in the ModelState response to provide useful error messages back to the client.

I have a request filter that does the following:

    public class InvalidModelStateFilterAttribute : ActionFilterAttribute {
        public override void OnActionExecuting(HttpActionContext actionContext) {
            if (!actionContext.ModelState.IsValid) {
                actionContext.Response = actionContext.Request.CreateErrorResponse(
                    HttpStatusCode.BadRequest, actionContext.ModelState);
            } else {
                base.OnActionExecuting(actionContext);
            }
        }
    }

Currently, that gets serialized to something like this:

{"Message":"The request is invalid.","ModelState":{"json.Name":["The category already exists."]}}

With Saule, I'm now getting something like this:

{"errors":[{"title":"The request is invalid."}]}

What would be necessary to change to have this errors array contain the actual fields and error descriptions?

[joukevandermaas]

Maybe this part of the documentation will help?

That generic error gets serialized if your response is of the type HttpError. There is basically no information in there, so Saule cannot serialize the error properly. If Saule somehow encounters an actual Exception object, it will produce more useful output.

I'm not 100% sure on how to do this using an HttpActionContext, but it should be possible.

[pdesjardins90]

I agree that model state validation should be support by the library, as it is a very common thing to do in an api. Here's what I did as a work around, maybe you can tweak this for you specific needs:

    public class ModelValidationAttribute : ActionFilterAttribute
    {
        public override void OnActionExecuting(HttpActionContext actionContext)
        {
            if (actionContext.ModelState.IsValid) return;

            var message = actionContext.ModelState
                .SelectMany(f => f.Value.Errors)
                .Aggregate("", (current, error) => current + error.ErrorMessage);            

            actionContext.Response = actionContext.Request
                .CreateErrorResponse(HttpStatusCode.BadRequest, message);
         }
    }

[rhyek]

Hi, hadn't checked this in a while. I ended up doing something similar:

        public override void OnActionExecuting(HttpActionContext actionContext) {
            if (!actionContext.ModelState.IsValid) {
                var modelState = actionContext.ModelState
                    .Select(p => new { key = p.Key, errors = p.Value.Errors.Select(e => e.ErrorMessage) })
                    .ToDictionary(kv => kv.key, kv => kv.errors);
                var json = JsonConvert.SerializeObject(new { ModelState = modelState });
                actionContext.Response = new HttpResponseMessage(HttpStatusCode.BadRequest) {
                    Content = new StringContent(json, Encoding.UTF8, "application/json")
                };
            } else {
                base.OnActionExecuting(actionContext);
            }
        }

This outputs json the same way WebAPI does it natively.

[triynko]

First of all, the key names are wrong in model state. It doesn't properly build the right path to the field, so it's useless. Build a validation utility to validate fields on the object yourself, and track proper key names. If you use request DTOs, you can even build in key resolution for naming strategies like 'snake_case'.

But even if it did have correct key names, running this select: Errors.Select(e => e.ErrorMessage) is not correct. In many case, ErrorMessage will be an empty string, and it's actually e.Exception.Message that you'd have to check, so you'd be better off doing something like this: Errors.Select(e => string.IsNullOrWhiteSpace(e.ErrorMessage) ? (e.Exception?.Message ?? "No error message.") : e.ErrorMessage).

[peymannj]

https://stackoverflow.com/questions/2845852/asp-net-mvc-how-to-convert-modelstate-errors-to-json/33888634