Insert type refinement for abstract lookups

Author: tobias-tengler

src/HotChocolate/Fusion-vnext/src/Fusion.Execution.Types/Metadata/Lookup.cs

@@ -65,6 +65,11 @@ public Lookup(
     /// </summary>
     public string Name { get; }
 
+    /// <summary>
+    /// Get the name of the declaring type.
+    /// </summary>
+    public string DeclaringTypeName => _declaringTypeName;
+
     /// <summary>
     /// Gets the arguments that represent field requirements.
     /// </summary>

src/HotChocolate/Fusion-vnext/src/Fusion.Execution/Planning/OperationPlanner.cs

@@ -295,6 +295,28 @@ lookup is null
                 requirements = requirements.Add(argumentRequirementKey, operationRequirement);
             }
 
+            // If the lookup returns an abstract type, we might need to insert a type refinement
+            // around our selection set.
+            if (_schema.Types.TryGetType(lookup.DeclaringTypeName, out var lookupDeclaringType)
+                && lookupDeclaringType != workItem.SelectionSet.Type)
+            {
+                var typeRefinement =
+                    new InlineFragmentNode(
+                        null,
+                        new NamedTypeNode(workItem.SelectionSet.Type.Name),
+                        [],
+                        resolvable);
+                var selectionSetWithTypeRefinement = new SelectionSetNode(null, [typeRefinement]);
+
+                var indexBuilder = index.ToBuilder();
+
+                indexBuilder.Register(resolvable, selectionSetWithTypeRefinement);
+
+                index = indexBuilder;
+
+                operationBuilder.SetSelectionSet(selectionSetWithTypeRefinement);
+            }
+
             operationBuilder.SetLookup(lookup, requirementKey);
         }
 

src/HotChocolate/Fusion-vnext/test/Fusion.Execution.Tests/Planning/__snapshots__/OperationPlannerTests.Plan_Simple_Interface_Lookup.yaml

@@ -1,13 +1,13 @@
 operation:
   - document: >-
-      query GetTopProducts {
-        topProduct {
-          id
-          name
-          price
-          id @fusion__requirement
-        }
+    query GetTopProducts {
+      topProduct {
+        id
+        name
+        price
+        id @fusion__requirement
       }
+    }
     name: GetTopProducts
     hash: 123
 nodes:
@@ -27,7 +27,9 @@ nodes:
         $__fusion_1_id: ID!
       ) {
         node(id: $__fusion_1_id) {
-          price
+          ... on Product {
+            price
+          }
         }
       }
     source: $.node

src/HotChocolate/Fusion-vnext/test/Fusion.Execution.Tests/Planning/__snapshots__/OperationPlannerTests.Plan_Simple_Union_Lookup.yaml

@@ -1,13 +1,13 @@
 operation:
   - document: >-
-      query GetTopProducts {
-        topProduct {
-          id
-          name
-          price
-          id @fusion__requirement
-        }
+    query GetTopProducts {
+      topProduct {
+        id
+        name
+        price
+        id @fusion__requirement
       }
+    }
     name: GetTopProducts
     hash: 123
 nodes:
@@ -27,7 +27,9 @@ nodes:
         $__fusion_1_id: ID!
       ) {
         lookupUnionById(id: $__fusion_1_id) {
-          price
+          ... on Product {
+            price
+          }
         }
       }
     source: $.lookupUnionById