Improve error response from invalid ModelState when [ApiController] is used

Author: bkoelman

src/JsonApiDotNetCore/Errors/UnsuccessfulActionResultException.cs

@@ -1,6 +1,7 @@
 using System.Net;
 using JetBrains.Annotations;
 using JsonApiDotNetCore.Serialization.Objects;
+using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Mvc;
 
 namespace JsonApiDotNetCore.Errors;
@@ -20,20 +21,35 @@ public UnsuccessfulActionResultException(HttpStatusCode status)
     }
 
     public UnsuccessfulActionResultException(ProblemDetails problemDetails)
-        : base(ToError(problemDetails))
+        : base(ToErrorObjects(problemDetails))
     {
     }
 
-    private static ErrorObject ToError(ProblemDetails problemDetails)
+    private static IEnumerable<ErrorObject> ToErrorObjects(ProblemDetails problemDetails)
     {
         ArgumentGuard.NotNull(problemDetails);
 
         HttpStatusCode status = problemDetails.Status != null ? (HttpStatusCode)problemDetails.Status.Value : HttpStatusCode.InternalServerError;
 
+        if (problemDetails is HttpValidationProblemDetails validationProblemDetails && validationProblemDetails.Errors.Any())
+        {
+            foreach (string errorMessage in validationProblemDetails.Errors.SelectMany(pair => pair.Value))
+            {
+                yield return ToErrorObject(status, validationProblemDetails, errorMessage);
+            }
+        }
+        else
+        {
+            yield return ToErrorObject(status, problemDetails, problemDetails.Detail);
+        }
+    }
+
+    private static ErrorObject ToErrorObject(HttpStatusCode status, ProblemDetails problemDetails, string? detail)
+    {
         var error = new ErrorObject(status)
         {
             Title = problemDetails.Title,
-            Detail = problemDetails.Detail
+            Detail = detail
         };
 
         if (!string.IsNullOrWhiteSpace(problemDetails.Instance))

test/JsonApiDotNetCoreTests/IntegrationTests/CustomRoutes/ApiControllerAttributeTests.cs

@@ -35,4 +35,48 @@ public async Task ApiController_attribute_transforms_NotFound_action_result_with
         error.Links.ShouldNotBeNull();
         error.Links.About.Should().Be("https://tools.ietf.org/html/rfc7231#section-6.5.4");
     }
+
+    [Fact]
+    public async Task ProblemDetails_from_invalid_ModelState_is_translated_into_error_response()
+    {
+        // Arrange
+        var requestBody = new
+        {
+            data = new
+            {
+                type = "civilians",
+                attributes = new
+                {
+                    name = (string?)null,
+                    yearOfBirth = 1850
+                }
+            }
+        };
+
+        const string route = "/world-civilians";
+
+        // Act
+        (HttpResponseMessage httpResponse, Document responseDocument) = await _testContext.ExecutePostAsync<Document>(route, requestBody);
+
+        // Assert
+        httpResponse.ShouldHaveStatusCode(HttpStatusCode.BadRequest);
+
+        responseDocument.Errors.ShouldHaveCount(2);
+
+        ErrorObject error1 = responseDocument.Errors[0];
+        error1.StatusCode.Should().Be(HttpStatusCode.BadRequest);
+        error1.Links.ShouldNotBeNull();
+        error1.Links.About.Should().Be("https://tools.ietf.org/html/rfc7231#section-6.5.1");
+        error1.Title.Should().Be("One or more validation errors occurred.");
+        error1.Detail.Should().Be("The Name field is required.");
+        error1.Source.Should().BeNull();
+
+        ErrorObject error2 = responseDocument.Errors[1];
+        error2.StatusCode.Should().Be(HttpStatusCode.BadRequest);
+        error2.Links.ShouldNotBeNull();
+        error2.Links.About.Should().Be("https://tools.ietf.org/html/rfc7231#section-6.5.1");
+        error2.Title.Should().Be("One or more validation errors occurred.");
+        error2.Detail.Should().Be("The field YearOfBirth must be between 1900 and 2050.");
+        error2.Source.Should().BeNull();
+    }
 }

test/JsonApiDotNetCoreTests/IntegrationTests/CustomRoutes/Civilian.cs

@@ -1,3 +1,4 @@
+using System.ComponentModel.DataAnnotations;
 using JetBrains.Annotations;
 using JsonApiDotNetCore.Resources;
 using JsonApiDotNetCore.Resources.Annotations;
@@ -10,4 +11,8 @@ public sealed class Civilian : Identifiable<int>
 {
     [Attr]
     public string Name { get; set; } = null!;
+
+    [Attr]
+    [Range(1900, 2050)]
+    public int YearOfBirth { get; set; }
 }