[Fusion] Correctly handle null items in ResolveByKey (#7217)

src/HotChocolate/Fusion/src/Core/Execution/Nodes/ResolveByKeyBatch.cs

@@ -258,7 +258,8 @@ private Dictionary<string, JsonElement> UnwrapResult(
 
             foreach (var element in data.EnumerateArray())
             {
-                if (element.TryGetProperty(key, out var keyValue))
+                if (element.ValueKind is not JsonValueKind.Null &&
+                    element.TryGetProperty(key, out var keyValue))
                 {
                     result.TryAdd(FormatKeyValue(keyValue), element);
                 }

src/HotChocolate/Fusion/test/Core.Tests/DemoIntegrationTests.cs

@@ -1861,6 +1861,59 @@ query TopProducts {
         Assert.Null(result.ExpectQueryResult().Errors);
     }
 
+    [Fact]
+    public async Task ResolveByKey_Handles_Null_Item_Correctly()
+    {
+        // arrange
+        using var demoProject = await DemoProject.CreateAsync();
+
+        // act
+        var fusionGraph = await new FusionGraphComposer(logFactory: _logFactory).ComposeAsync(
+            new[]
+            {
+                demoProject.Products.ToConfiguration(),
+                demoProject.Resale.ToConfiguration(),
+            }, new FusionFeatureCollection(FusionFeatures.NodeField));
+
+        var executor = await new ServiceCollection()
+            .AddSingleton(demoProject.HttpClientFactory)
+            .AddSingleton(demoProject.WebSocketConnectionFactory)
+            .AddFusionGatewayServer()
+            .ConfigureFromDocument(SchemaFormatter.FormatAsDocument(fusionGraph))
+            .BuildRequestExecutorAsync();
+
+        var request = Parse(
+            """
+            {
+              viewer {
+                # The second product does not exist in the products subgraph
+                recommendedResalableProducts {
+                  edges {
+                    node {
+                      product? {
+                        id
+                        name
+                      }
+                    }
+                  }
+                }
+              }
+            }
+            """);
+
+        // act
+        await using var result = await executor.ExecuteAsync(
+            OperationRequestBuilder
+                .New()
+                .SetDocument(request)
+                .Build());
+
+        // assert
+        var snapshot = new Snapshot();
+        CollectSnapshotData(snapshot, request, result);
+        await snapshot.MatchMarkdownAsync();
+    }
+
     public sealed class HotReloadConfiguration : IObservable<GatewayConfiguration>
     {
         private GatewayConfiguration _configuration;

src/HotChocolate/Fusion/test/Core.Tests/__snapshots__/DemoIntegrationTests.ResolveByKey_Handles_Null_Item_Correctly.md

@@ -0,0 +1,139 @@
+# ResolveByKey_Handles_Null_Item_Correctly
+
+## Result
+
+```json
+{
+  "errors": [
+    {
+      "message": "Cannot return null for non-nullable field.",
+      "locations": [
+        {
+          "line": 9,
+          "column": 13
+        }
+      ],
+      "path": [
+        "viewer",
+        "recommendedResalableProducts",
+        "edges",
+        1,
+        "node",
+        "product",
+        "name"
+      ],
+      "extensions": {
+        "code": "HC0018"
+      }
+    }
+  ],
+  "data": {
+    "viewer": {
+      "recommendedResalableProducts": {
+        "edges": [
+          {
+            "node": {
+              "product": {
+                "id": "UHJvZHVjdDox",
+                "name": "Table"
+              }
+            }
+          },
+          {
+            "node": {
+              "product": null
+            }
+          },
+          {
+            "node": {
+              "product": {
+                "id": "UHJvZHVjdDoz",
+                "name": "Chair"
+              }
+            }
+          }
+        ]
+      }
+    }
+  }
+}
+```
+
+## Request
+
+```graphql
+{
+  viewer {
+    recommendedResalableProducts {
+      edges {
+        node {
+          product? {
+            id
+            name
+          }
+        }
+      }
+    }
+  }
+}
+```
+
+## QueryPlan Hash
+
+```text
+143888B7D930092D26863B1CCBEFA6A7A06739E8
+```
+
+## QueryPlan
+
+```json
+{
+  "document": "{ viewer { recommendedResalableProducts { edges { node { product? { id name } } } } } }",
+  "rootNode": {
+    "type": "Sequence",
+    "nodes": [
+      {
+        "type": "Resolve",
+        "subgraph": "Resale",
+        "document": "query fetch_viewer_1 { viewer { recommendedResalableProducts { edges { node { product? { id __fusion_exports__1: id } } } } } }",
+        "selectionSetId": 0,
+        "provides": [
+          {
+            "variable": "__fusion_exports__1"
+          }
+        ]
+      },
+      {
+        "type": "Compose",
+        "selectionSetIds": [
+          0
+        ]
+      },
+      {
+        "type": "ResolveByKeyBatch",
+        "subgraph": "Products",
+        "document": "query fetch_viewer_2($__fusion_exports__1: [ID!]!) { nodes(ids: $__fusion_exports__1) { ... on Product { name __fusion_exports__1: id } } }",
+        "selectionSetId": 5,
+        "path": [
+          "nodes"
+        ],
+        "requires": [
+          {
+            "variable": "__fusion_exports__1"
+          }
+        ]
+      },
+      {
+        "type": "Compose",
+        "selectionSetIds": [
+          5
+        ]
+      }
+    ]
+  },
+  "state": {
+    "__fusion_exports__1": "Product_id"
+  }
+}
+```
+

src/HotChocolate/Fusion/test/Shared/DemoProject.cs

@@ -8,6 +8,7 @@
 using HotChocolate.Fusion.Shared.Shipping;
 using HotChocolate.Fusion.Shared.Books;
 using HotChocolate.Fusion.Shared.Authors;
+using HotChocolate.Fusion.Shared.Resale;
 using HotChocolate.Transport.Http;
 using HotChocolate.Types.Descriptors;
 using HotChocolate.Utilities.Introspection;
@@ -33,6 +34,7 @@ private DemoProject(
         DemoSubgraph patient1,
         DemoSubgraph books,
         DemoSubgraph authors,
+        DemoSubgraph resale,
         IHttpClientFactory clientFactory,
         IWebSocketConnectionFactory webSocketConnectionFactory)
     {
@@ -46,6 +48,7 @@ private DemoProject(
         Patient1 = patient1;
         Books = books;
         Authors = authors;
+        Resale = resale;
         HttpClientFactory = clientFactory;
         WebSocketConnectionFactory = webSocketConnectionFactory;
     }
@@ -72,6 +75,8 @@ private DemoProject(
 
     public DemoSubgraph Authors { get; }
 
+    public DemoSubgraph Resale { get; }
+
     public static async Task<DemoProject> CreateAsync(CancellationToken ct = default)
     {
         var disposables = new List<IDisposable>();
@@ -257,6 +262,26 @@ public static async Task<DemoProject> CreateAsync(CancellationToken ct = default
             .IntrospectServerAsync(authorsClient, ct)
             .ConfigureAwait(false);
 
+        var resale = testServerFactory.Create(
+            s => s
+                .AddRouting()
+                .AddGraphQLServer(disableCostAnalyzer: true)
+                .AddQueryType<ResaleQuery>()
+                .AddGlobalObjectIdentification()
+                .AddMutationConventions()
+                .AddUploadType()
+                .AddConvention<INamingConventions>(_ => new DefaultNamingConventions()),
+            c => c
+                .UseRouting()
+                .UseEndpoints(endpoints => endpoints.MapGraphQL()));
+        disposables.Add(resale);
+
+        var resaleClient = resale.CreateClient();
+        resaleClient.BaseAddress = new Uri("http://localhost:5000/graphql");
+        var resaleSchema = await IntrospectionClient
+            .IntrospectServerAsync(resaleClient, ct)
+            .ConfigureAwait(false);
+
         var httpClients = new Dictionary<string, Func<HttpClient>>
         {
             {
@@ -349,6 +374,16 @@ public static async Task<DemoProject> CreateAsync(CancellationToken ct = default
                     return httpClient;
                 }
             },
+            {
+                "Resale", () =>
+                {
+                    // ReSharper disable once AccessToDisposedClosure
+                    var httpClient = resale.CreateClient();
+                    httpClient.BaseAddress = new Uri("http://localhost:5000/graphql");
+                    httpClient.DefaultRequestHeaders.AddGraphQLPreflight();
+                    return httpClient;
+                }
+            },
         };
 
         var webSocketClients = new Dictionary<string, Func<IWebSocketConnection>>
@@ -380,6 +415,9 @@ public static async Task<DemoProject> CreateAsync(CancellationToken ct = default
             {
                 "Authors", () => new MockWebSocketConnection(authors.CreateWebSocketClient())
             },
+            {
+                "Resale", () => new MockWebSocketConnection(resale.CreateWebSocketClient())
+            },
         };
 
         return new DemoProject(
@@ -438,6 +476,12 @@ public static async Task<DemoProject> CreateAsync(CancellationToken ct = default
                 new Uri("ws://localhost:5000/graphql"),
                 authorsSchema,
                 authors),
+            new DemoSubgraph(
+                "Resale",
+                resaleClient.BaseAddress,
+                new Uri("ws://localhost:5000/graphql"),
+                resaleSchema,
+                resale),
             new MockHttpClientFactory(httpClients),
             new MockWebSocketConnectionFactory(webSocketClients));
     }

src/HotChocolate/Fusion/test/Shared/Resale/ResaleQuery.cs

@@ -0,0 +1,32 @@
+using HotChocolate.Types;
+using HotChocolate.Types.Relay;
+
+namespace HotChocolate.Fusion.Shared.Resale;
+
+[GraphQLName("Query")]
+public class ResaleQuery
+{
+    public Viewer GetViewer() => new();
+
+    [NodeResolver]
+    public Product? GetProductById([ID] int id) => new(id);
+}
+
+public class Viewer
+{
+    [UsePaging]
+    public List<RecommendedResalableProduct> GetRecommendedResalableProducts()
+    {
+        return new()
+        {
+            new RecommendedResalableProduct(new Product(1)),
+            // This product doesn't exist on the products subgraph
+            new RecommendedResalableProduct(new Product(5)),
+            new RecommendedResalableProduct(new Product(3))
+        };
+    }
+}
+
+public record RecommendedResalableProduct(Product Product);
+
+public record Product([property: ID] int Id);