Added missing test for error case

Author: Bart Koelman

src/JsonApiDotNetCore/QueryParameterServices/SparseFieldsService.cs

@@ -130,11 +130,9 @@ private void RegisterRelatedResourceFields(IEnumerable<string> fields, Relations
                 var attr = relationProperty.Attributes.SingleOrDefault(a => a.Is(field));
                 if (attr == null)
                 {
-                    // TODO: Add unit test for this error, once the nesting limitation is removed and this code becomes reachable again.
-
                     throw new InvalidQueryStringParameterException(parameterName,
-                        "Sparse field navigation path refers to an invalid related field.",
-                        $"Related resource '{relationship.RightType.Name}' does not contain an attribute named '{field}'.");
+                        "The specified field does not exist on the requested related resource.",
+                        $"The field '{field}' does not exist on related resource '{relationship.PublicRelationshipName}' of type '{relationProperty.ResourceName}'.");
                 }
 
                 if (attr.PropertyInfo.SetMethod == null)

test/UnitTests/QueryParameters/SparseFieldsServiceTests.cs

@@ -7,6 +7,7 @@
 using JsonApiDotNetCoreExample.Models;
 using Microsoft.Extensions.Primitives;
 using Xunit;
+using Person = UnitTests.TestModels.Person;
 
 namespace UnitTests.QueryParameters
 {
@@ -162,6 +163,40 @@ public void Parse_InvalidField_ThrowsJsonApiException()
             Assert.Equal("fields", exception.Error.Source.Parameter);
         }
 
+        [Fact]
+        public void Parse_InvalidRelatedField_ThrowsJsonApiException()
+        {
+            // Arrange
+            var idAttribute = new AttrAttribute("id") {PropertyInfo = typeof(Article).GetProperty(nameof(Article.Id))};
+
+            var query = new KeyValuePair<string, StringValues>("fields[author]", "invalid");
+
+            var resourceContext = new ResourceContext
+            {
+                ResourceName = "articles",
+                Attributes = new List<AttrAttribute> {idAttribute},
+                Relationships = new List<RelationshipAttribute>
+                {
+                    new HasOneAttribute("author")
+                    {
+                        PropertyInfo = typeof(Article).GetProperty(nameof(Article.Author)),
+                        RightType = typeof(Person)
+                    }
+                }
+            };
+
+            var service = GetService(resourceContext);
+
+            // Act, assert
+            var exception = Assert.Throws<InvalidQueryStringParameterException>(() => service.Parse(query.Key, query.Value));
+            
+            Assert.Equal("fields[author]", exception.QueryParameterName);
+            Assert.Equal(HttpStatusCode.BadRequest, exception.Error.StatusCode);
+            Assert.Equal("The specified field does not exist on the requested related resource.", exception.Error.Title);
+            Assert.Equal("The field 'invalid' does not exist on related resource 'author' of type 'people'.", exception.Error.Detail);
+            Assert.Equal("fields[author]", exception.Error.Source.Parameter);
+        }
+
         [Fact]
         public void Parse_LegacyNotation_ThrowsJsonApiException()
         {