Update implementation of OpenApiWorkspace.ResolveReference to support referencing document fragments

Author: simon-pearson-lmk

src/Microsoft.OpenApi/Services/OpenApiWorkspace.cs

@@ -92,7 +92,7 @@ public void AddDocument(string location, OpenApiDocument  document)
         /// <remarks>Not sure how this is going to work.  Does the reference just point to the fragment as a whole, or do we need to 
         /// to be able to point into the fragment.  Keeping it private until we figure it out.
         /// </remarks>
-        private void AddFragment(string location, IOpenApiElement fragment)
+        public void AddFragment(string location, IOpenApiElement fragment)
         {
             _fragments.Add(ToLocationUrl(location), fragment);
         }
@@ -114,21 +114,14 @@ public void AddArtifact(string location, Stream artifact)
         /// <returns></returns>
         public IOpenApiReferenceable ResolveReference(OpenApiReference reference)
         {
-            if (_documents.TryGetValue(new Uri(BaseUrl,reference.ExternalResource),out var doc))
+            if (_documents.TryGetValue(new Uri(BaseUrl, reference.ExternalResource), out var doc))
             {
-                return doc.ResolveReference(reference, true); 
+                return doc.ResolveReference(reference, true);
             }
             else if (_fragments.TryGetValue(new Uri(BaseUrl, reference.ExternalResource), out var fragment))
             {
-                var frag = fragment as IOpenApiReferenceable;
-                if (frag != null)
-                {
-                    return null; // frag.ResolveReference(reference, true); // IOpenApiElement needs to implement ResolveReference
-                }
-                else
-                {
-                    return null;
-                }
+                var pointer = new JsonPointer($"/{reference.Id ?? string.Empty}");
+                return fragment.ResolveReference(pointer);
             }
             return null;
         }

test/Microsoft.OpenApi.Tests/Workspaces/OpenApiWorkspaceTests.cs

@@ -155,6 +155,51 @@ public void OpenApiWorkspacesShouldLoadDocumentFragments()
             Assert.True(false);
         }
 
+        [Fact]
+        public void OpenApiWorkspacesCanResolveReferencesToDocumentFragments()
+        {
+            // Arrange
+            var workspace = new OpenApiWorkspace();
+            var schemaFragment = new OpenApiSchema { Type = "string", Description = "Schema from a fragment" };
+            workspace.AddFragment("fragment", schemaFragment);
+
+            // Act
+            var schema = workspace.ResolveReference(new OpenApiReference()
+            {
+                ExternalResource = "fragment"
+            }) as OpenApiSchema;
+
+            // Assert
+            Assert.NotNull(schema);
+            Assert.Equal("Schema from a fragment", schema.Description);
+        }
+
+        [Fact]
+        public void OpenApiWorkspacesCanResolveReferencesToDocumentFragmentsWithJsonPointers()
+        {
+            // Arrange
+            var workspace = new OpenApiWorkspace();
+            var componentsFragment = new OpenApiComponents
+            {
+                Schemas =
+                {
+                    ["schemaOne"] = new OpenApiSchema() {Type = "string", Description = "Schema from a fragment"}
+                }
+            };
+            workspace.AddFragment("fragment", componentsFragment);
+
+            // Act
+            var schema = workspace.ResolveReference(new OpenApiReference()
+            {
+                Id = "components/schemas/schemaOne",
+                ExternalResource = "fragment"
+            }) as OpenApiSchema;
+
+            // Assert
+            Assert.NotNull(schema);
+            Assert.Equal("Schema from a fragment", schema.Description);
+        }
+
 
         // Test artifacts