fix: sub-schema references are invalid

src/Microsoft.OpenApi/Models/BaseOpenApiReference.cs

@@ -55,7 +55,7 @@
         /// <summary>
         /// The OpenApiDocument that is hosting the OpenApiReference instance. This is used to enable dereferencing the reference.
         /// </summary>
         public OpenApiDocument? HostDocument { get => hostDocument; init => hostDocument = value; }
 
         private string? _referenceV3;
         /// <summary>
@@ -84,8 +84,14 @@
                 {
                     return Id;
                 }
-                if (!string.IsNullOrEmpty(Id) && Id is not null && Id.StartsWith("http://", StringComparison.OrdinalIgnoreCase) ||
-                    !string.IsNullOrEmpty(Id) && Id is not null && Id.StartsWith("https://", StringComparison.OrdinalIgnoreCase))
+                if (!string.IsNullOrEmpty(Id) && Id is not null &&
+                    (Id.StartsWith("http://", StringComparison.OrdinalIgnoreCase) ||
+                     Id.StartsWith("https://", StringComparison.OrdinalIgnoreCase) ||
+#if NETSTANDARD2_1 || NETCOREAPP2_1_OR_GREATER || NET5_0_OR_GREATER
+                     Id.Contains("#/components", StringComparison.OrdinalIgnoreCase)))
+#else
+                     Id.Contains("#/components")))
+#endif
                 {
                     return Id;
                 }

test/Microsoft.OpenApi.Readers.Tests/V31Tests/ReferenceSamples/STJSchema.json

@@ -0,0 +1,67 @@
+{
+  "type": "object",
+  "properties": {
+    "name": {
+      "type": [
+        "string",
+        "null"
+      ],
+      "format": null,
+      "x-schema-id": null
+    },
+    "parent": {
+      "type": [
+        "object",
+        "null"
+      ],
+      "properties": {
+        "name": {
+          "type": [
+            "string",
+            "null"
+          ],
+          "format": null,
+          "x-schema-id": null
+        },
+        "parent": {
+          "$ref": "#/properties/parent",
+          "x-schema-id": "Category"
+        },
+        "tags": {
+          "type": [
+            "array",
+            "null"
+          ],
+          "items": {
+            "type": "object",
+            "properties": {
+              "name": {
+                "type": [
+                  "string",
+                  "null"
+                ],
+                "format": null,
+                "x-schema-id": null
+              }
+            },
+            "required": [
+              "name"
+            ],
+            "x-schema-id": "Tag"
+          }
+        }
+      },
+      "required": [
+        "name"
+      ],
+      "x-schema-id": "Category"
+    },
+    "tags": {
+      "$ref": "#/properties/parent/properties/tags"
+    }
+  },
+  "required": [
+    "name"
+  ],
+  "x-schema-id": "Category"
+}

test/Microsoft.OpenApi.Readers.Tests/V31Tests/RelativeReferenceTests.cs

@@ -312,7 +312,7 @@ public async Task ShouldResolveRelativeSubReferenceUsingParsingContext()
             parsingContext.StartObject("schemas");
             parsingContext.StartObject("Foo");
             var document = new OpenApiDocument();
-            
+
             // Act
             var fooComponentSchema = parsingContext.ParseFragment<OpenApiSchema>(schemaJsonNode, OpenApiSpecVersion.OpenApi3_1, document);
             document.AddComponent("Foo", fooComponentSchema);
@@ -412,5 +412,28 @@ public async Task ShouldResolveRecursiveRelativeSubReference()
             Assert.Equal(JsonSchemaType.Array | JsonSchemaType.Null, fooSchemaTagsProperty.Type);
             Assert.Equal(JsonSchemaType.Object, fooSchemaTagsProperty.Items.Type);
         }
+        [Fact]
+        public async Task ShouldResolveReferencesInSchemasFromSystemTextJson()
+        {
+            var filePath = Path.Combine(SampleFolderPath, "STJSchema.json");
+            using var fs = File.OpenRead(filePath);
+            var jsonNode = await JsonNode.ParseAsync(fs);
+
+            var parsingContext = new ParsingContext(new OpenApiDiagnostic());
+            var document = new OpenApiDocument();
+            var schema = parsingContext.ParseFragment<OpenApiSchema>(jsonNode, OpenApiSpecVersion.OpenApi3_1, document);
+            Assert.NotNull(schema);
+
+            document.AddComponent("Foo", schema);
+            var tagsProperty = Assert.IsType<OpenApiSchemaReference>(schema.Properties["tags"]);
+            // this is the reference that is generated by STJ schema generator which does not have OAI in context.
+            Assert.Equal("#/properties/parent/properties/tags", tagsProperty.Reference.ReferenceV3);
+            // this is the reference that needs to be used in the document for components resolution.
+            var absoluteReferenceId = $"#/components/schemas/Foo{tagsProperty.Reference.ReferenceV3.Replace("#", string.Empty)}";
+            schema.Properties["tags"] = new OpenApiSchemaReference(absoluteReferenceId, document);
+            var updatedTagsProperty = Assert.IsType<OpenApiSchemaReference>(schema.Properties["tags"]);
+            Assert.Equal(absoluteReferenceId, updatedTagsProperty.Reference.ReferenceV3);
+            Assert.Equal(JsonSchemaType.Array | JsonSchemaType.Null, updatedTagsProperty.Type);
+        }
     }
 }