Determine providing selection set

Author: tobias-tengler

src/HotChocolate/Fusion-vnext/src/Fusion.Execution/Execution/Nodes/OperationExecutionNode.cs

@@ -55,6 +55,57 @@ public OperationExecutionNode(
         }
 
         _variables = variables.ToArray();
+
+        var selectionSet = GetSelectionSetNodeFromPath(operation, source);
+
+        _providingSelectionSet = selectionSet ?? throw new InvalidOperationException("Could not determine source selection set");
+    }
+
+    private readonly SelectionSetNode _providingSelectionSet;
+
+    // TODO: Move
+    private static SelectionSetNode? GetSelectionSetNodeFromPath(OperationDefinitionNode operationDefinition, SelectionPath path)
+    {
+        var current = operationDefinition.SelectionSet;
+
+        if (path.IsRoot)
+        {
+            return current;
+        }
+
+        for (var i = path.Segments.Length - 1; i >= 0; i--)
+        {
+            var segment = path.Segments[i];
+
+            if (segment.Kind == SelectionPathSegmentKind.InlineFragment)
+            {
+                // TODO: Do we need to handle the case without a type condition?
+                var selection = current.Selections
+                    .OfType<InlineFragmentNode>()
+                    .FirstOrDefault(s => s.TypeCondition?.Name.Value == segment.Name);
+
+                if (selection is null)
+                {
+                    return null;
+                }
+
+                current = selection.SelectionSet;
+            }
+            else if (segment.Kind is SelectionPathSegmentKind.Field)
+            {
+                var selection = current.Selections
+                    .OfType<FieldNode>()
+                    .FirstOrDefault(s => s.Alias?.Value == segment.Name || s.Name.Value == segment.Name);
+
+                if (selection?.SelectionSet is null)
+                {
+                    return null;
+                }
+                current = selection.SelectionSet;
+            }
+        }
+
+        return current;
     }
 
     public override int Id { get; }

src/HotChocolate/Fusion-vnext/test/Fusion.AspNetCore.Tests/SubgraphErrorTests.cs

@@ -304,6 +304,44 @@ public async Task No_Data_And_Error_With_Path_For_Lookup()
         response.MatchSnapshot();
     }
 
+    [Fact]
+    public async Task No_Data_And_Error_With_Path_For_Lookup_Leaf()
+    {
+        // arrange
+        using var server1 = CreateSourceSchema(
+            "A",
+            b => b.AddQueryType<SourceSchema1.Query>());
+
+        using var server2 = CreateSourceSchema(
+            "B",
+            b => b.AddQueryType<SourceSchema6.Query>());
+
+        // act
+        using var gateway = await CreateCompositeSchemaAsync(
+        [
+            ("A", server1),
+            ("B", server2),
+        ]);
+
+        // assert
+        using var client = GraphQLHttpClient.Create(gateway.CreateClient());
+
+        using var result = await client.PostAsync(
+            """
+            {
+              topProduct {
+                price
+                name
+              }
+            }
+            """,
+            new Uri("http://localhost:5000/graphql"));
+
+        // act
+        using var response = await result.ReadAsResultAsync();
+        response.MatchSnapshot();
+    }
+
     [Fact]
     public async Task No_Data_And_Error_Without_Path_For_Lookup()
     {
@@ -475,4 +513,22 @@ public record Product(int Id)
             public string GetName() => "Product " + Id;
         }
     }
+
+    public static class SourceSchema6
+    {
+        public class Query
+        {
+            [Lookup]
+            public Product GetProductById(int id) => new(id);
+        }
+
+        public record Product(int Id)
+        {
+            public string GetName(IResolverContext context)
+            {
+                throw new GraphQLException(ErrorBuilder.New().SetMessage("Could not resolve Product.name")
+                    .SetPath(context.Path).Build());
+            }
+        }
+    }
 }

src/HotChocolate/Fusion-vnext/test/Fusion.AspNetCore.Tests/__snapshots__/SubgraphErrorTests.No_Data_And_Error_With_Path_For_Lookup_Leaf.snap

@@ -0,0 +1,52 @@
+{
+  "data": {
+    "topProduct": {
+      "price": 13.99,
+      "name": null
+    }
+  },
+  "errors": [
+    {
+      "message": "Could not resolve Product.name",
+      "path": [
+        "topProduct",
+        "name"
+      ]
+    }
+  ],
+  "extensions": {
+    "fusion": {
+      "operationPlan": {
+        "operation": {
+          "document": "{\n  topProduct {\n    price\n    name\n    id @fusion__requirement\n  }\n}",
+          "hash": "e24d93b6245cedd878fff4452f5f16b9"
+        },
+        "nodes": [
+          {
+            "id": 1,
+            "type": "Operation",
+            "schema": "A",
+            "operation": "query Op_e24d93b6_1 {\n  topProduct {\n    price\n    id\n  }\n}"
+          },
+          {
+            "id": 2,
+            "type": "Operation",
+            "schema": "B",
+            "operation": "query Op_e24d93b6_2(\n  $__fusion_1_id: Int!\n) {\n  productById(id: $__fusion_1_id) {\n    name\n  }\n}",
+            "source": "$.productById",
+            "target": "$.topProduct",
+            "requirements": [
+              {
+                "name": "__fusion_1_id",
+                "selectionMap": "id"
+              }
+            ],
+            "dependencies": [
+              1
+            ]
+          }
+        ]
+      }
+    }
+  }
+}