Fix handle single-type abstract selection in AsSelector

src/HotChocolate/Core/src/Execution.Projections/SelectionExpressionBuilder.cs

@@ -165,7 +165,28 @@ private void CollectTypes(Context context, Selection selection, TypeContainer pa
             return switchExpression;
         }
 
-        return BuildSelectionSetExpression(context, parent.Nodes[0]);
+        var singleTypeNode = parent.Nodes[0];
+
+        if (context.ParentType != singleTypeNode.Type)
+        {
+            var newParent = Expression.Convert(context.Parent, singleTypeNode.Type);
+            var newContext = context with { Parent = newParent, ParentType = singleTypeNode.Type };
+            var selectionSet = BuildSelectionSetExpression(newContext, singleTypeNode);
+
+            if (selectionSet is null)
+            {
+                return null;
+            }
+
+            var castedSelectionSet = Expression.Convert(selectionSet, context.ParentType);
+
+            return Expression.Condition(
+                Expression.TypeIs(context.Parent, singleTypeNode.Type),
+                castedSelectionSet,
+                Expression.Constant(null, context.ParentType));
+        }
+
+        return BuildSelectionSetExpression(context, singleTypeNode);
     }
 
     private static MemberInitExpression? BuildSelectionSetExpression(

src/HotChocolate/Data/test/Data.Projections.SqlServer.Tests/QueryableProjectionUnionTypeTests.cs

@@ -39,6 +39,28 @@ public class QueryableProjectionUnionTypeTests
         }
     ];
 
+    private static readonly InspectionDefinition[] s_inspectionDefinitions =
+    [
+        new()
+        {
+            Id = 1,
+            Trigger = new FieldDateTimeInspectionTrigger
+            {
+                Id = 11,
+                FieldModelKey = "field-1"
+            }
+        },
+        new()
+        {
+            Id = 2,
+            Trigger = new FieldDateTimeInspectionTrigger
+            {
+                Id = 12,
+                FieldModelKey = "field-2"
+            }
+        }
+    ];
+
     private readonly SchemaCache _cache = new SchemaCache();
 
     [Fact]
@@ -214,6 +236,75 @@ await Snapshot
             .MatchAsync();
     }
 
+    [Fact]
+    public async Task Create_Union_Single_Property()
+    {
+        // arrange
+        var tester = _cache.CreateSchema(
+            s_inspectionDefinitions,
+            OnModelCreatingInspection,
+            configure: ConfigureInspectionSchema);
+
+        // act
+        var result = await tester.ExecuteAsync(
+            OperationRequestBuilder.New()
+                .SetDocument(
+                    """
+                    {
+                        root {
+                            trigger {
+                                ... on FieldDateTimeInspectionTrigger {
+                                    fieldModelKey
+                                }
+                            }
+                        }
+                    }
+                    """)
+                .Build());
+
+        // assert
+        await Snapshot
+            .Create()
+            .AddResult(result)
+            .MatchAsync();
+    }
+
+    [Fact]
+    public async Task Create_Union_Single_Property_Pagination()
+    {
+        // arrange
+        var tester = _cache.CreateSchema(
+            s_inspectionDefinitions,
+            OnModelCreatingInspection,
+            usePaging: true,
+            configure: ConfigureInspectionSchema);
+
+        // act
+        var result = await tester.ExecuteAsync(
+            OperationRequestBuilder.New()
+                .SetDocument(
+                    """
+                    {
+                        root {
+                            nodes {
+                                trigger {
+                                    ... on FieldDateTimeInspectionTrigger {
+                                        fieldModelKey
+                                    }
+                                }
+                            }
+                        }
+                    }
+                    """)
+                .Build());
+
+        // assert
+        await Snapshot
+            .Create()
+            .AddResult(result)
+            .MatchAsync();
+    }
+
     private static void OnModelCreating(ModelBuilder modelBuilder)
     {
         modelBuilder.Entity<AbstractType>()
@@ -229,6 +320,21 @@ private static void ConfigureSchema(ISchemaBuilder schemaBuilder)
             .AddType(new ObjectType<Bar>());
     }
 
+    private static void OnModelCreatingInspection(ModelBuilder modelBuilder)
+    {
+        modelBuilder.Entity<InspectionDefinition>()
+            .HasOne(x => x.Trigger)
+            .WithOne()
+            .HasForeignKey<InspectionDefinition>(x => x.TriggerId);
+
+        modelBuilder.Entity<InspectionTrigger>()
+            .HasDiscriminator<string>("d")
+            .HasValue<FieldDateTimeInspectionTrigger>("fieldDateTime");
+    }
+
+    private static void ConfigureInspectionSchema(ISchemaBuilder schemaBuilder)
+        => schemaBuilder.AddType(new ObjectType<FieldDateTimeInspectionTrigger>());
+
     public class NestedList
     {
         public int Id { get; set; }
@@ -260,4 +366,24 @@ public class Bar : AbstractType
     {
         public string BarProp { get; set; } = null!;
     }
+
+    public class InspectionDefinition
+    {
+        public int Id { get; set; }
+
+        public int TriggerId { get; set; }
+
+        public InspectionTrigger Trigger { get; set; } = null!;
+    }
+
+    [UnionType]
+    public abstract class InspectionTrigger
+    {
+        public int Id { get; set; }
+    }
+
+    public class FieldDateTimeInspectionTrigger : InspectionTrigger
+    {
+        public string FieldModelKey { get; set; } = null!;
+    }
 }

src/HotChocolate/Data/test/Data.Projections.SqlServer.Tests/__snapshots__/QueryableProjectionUnionTypeTests.Create_Union_Single_Property.snap

@@ -0,0 +1,26 @@
+Result:
+---------------
+{
+  "data": {
+    "root": [
+      {
+        "trigger": {
+          "fieldModelKey": "field-1"
+        }
+      },
+      {
+        "trigger": {
+          "fieldModelKey": "field-2"
+        }
+      }
+    ]
+  }
+}
+---------------
+
+SQL:
+---------------
+SELECT 1, 1, "i"."FieldModelKey"
+FROM "Data" AS "d"
+INNER JOIN "InspectionTrigger" AS "i" ON "d"."TriggerId" = "i"."Id"
+---------------

src/HotChocolate/Data/test/Data.Projections.SqlServer.Tests/__snapshots__/QueryableProjectionUnionTypeTests.Create_Union_Single_Property_Pagination.snap

@@ -0,0 +1,28 @@
+Result:
+---------------
+{
+  "data": {
+    "root": {
+      "nodes": [
+        {
+          "trigger": {
+            "fieldModelKey": "field-1"
+          }
+        },
+        {
+          "trigger": {
+            "fieldModelKey": "field-2"
+          }
+        }
+      ]
+    }
+  }
+}
+---------------
+
+SQL:
+---------------
+SELECT 1, 1, "i"."FieldModelKey"
+FROM "Data" AS "d"
+INNER JOIN "InspectionTrigger" AS "i" ON "d"."TriggerId" = "i"."Id"
+---------------