Unmarshal json null relationship data to nil

jsonapi_test.go

@@ -173,6 +173,7 @@ var (
 	articleRelatedInvalidEmptyRelationshipBody  = `{"data":{"id":"1","type":"articles","attributes":{"title":"A"},"relationships":{"author":{}}}}`
 	articleRelatedInvalidEmptyDataBody          = `{"data":{"id":"1","type":"articles","attributes":{"title":"A"},"relationships":{"author":{"data":{}}}}}`
 	articleRelatedNoOmitEmptyBody               = `{"data":{"id":"1","type":"articles","attributes":{"title":"A"},"relationships":{"author":{"data":null},"comments":{"data":[]}}}}`
+	articleRelatedLiteralNullData               = `{"data":{"id":"1","type":"articles","attributes":{"title":"A"},"relationships":{"author":{"data":null},"comments":{"data":null}}}}`
 	articleRelatedAuthorBody                    = `{"data":{"id":"1","type":"articles","attributes":{"title":"A"},"relationships":{"author":{"data":{"id":"1","type":"author"},"links":{"self":"http://example.com/articles/1/relationships/author","related":"http://example.com/articles/1/author"}}}}}`
 	articleRelatedAuthorTwiceBody               = `{"data":[{"id":"1","type":"articles","attributes":{"title":"A"},"relationships":{"author":{"data":{"id":"1","type":"author"}}}},{"id":"2","type":"articles","attributes":{"title":"B"},"relationships":{"author":{"data":{"id":"1","type":"author"}}}}]}`
 	articleRelatedAuthorTwiceWithIncludeBody    = `{"data":[{"id":"1","type":"articles","attributes":{"title":"A"},"relationships":{"author":{"data":{"id":"1","type":"author"}}}},{"id":"2","type":"articles","attributes":{"title":"B"},"relationships":{"author":{"data":{"id":"1","type":"author"}}}}],"included":[{"id":"1","type":"author","attributes":{"name":"A"}}]}`

reflect.go

@@ -47,3 +47,10 @@ func setFieldValue(fv reflect.Value, v any) {
 	}
 	fv.Set(vv)
 }
+
+func canBeNil(fv reflect.Value) bool {
+	return fv.Kind() == reflect.Interface ||
+		fv.Kind() == reflect.Map ||
+		fv.Kind() == reflect.Pointer ||
+		fv.Kind() == reflect.Slice
+}

unmarshal.go

@@ -293,6 +293,9 @@ func (ro *resourceObject) unmarshalFields(v any, rv reflect.Value, rt reflect.Ty
 			}
 			if !relDocument.hasMany && relDocument.isEmpty() {
 				// ensure struct field is nil for data:null cases only (we want empty slice for data:[])
+				if canBeNil(fv) {
+					fv.Set(reflect.Zero(ft.Type))
+				}
 				continue
 			}
 

unmarshal_test.go

@@ -330,6 +330,19 @@ func TestUnmarshal(t *testing.T) {
 			},
 			expect:      &ArticleRelated{},
 			expectError: ErrEmptyDataObject,
+		}, {
+			description: "*ArticleRelated with nil relationships data",
+			given:       articleRelatedLiteralNullData,
+			do: func(body []byte) (any, error) {
+				a := ArticleRelated{
+					Author:   &Author{ID: "100"},
+					Comments: []*Comment{{ID: "200"}},
+				}
+				err := Unmarshal(body, &a)
+				return &a, err
+			},
+			expect:      &ArticleRelated{ID: "1", Title: "A", Author: nil, Comments: nil},
+			expectError: nil,
 		}, {
 			description: "*ArticleRelated.Author",
 			given:       articleRelatedAuthorBody,