Projections: Support selecting only __typename on union types

src/HotChocolate/Data/src/Data/Projections/Expressions/QueryableProjectionScopeExtensions.cs

@@ -35,12 +35,25 @@ public static Expression CreateMemberInit(this QueryableProjectionScope scope)
 
             foreach (var val in scope.GetAbstractTypes())
             {
-                var ctor = Expression.New(val.Key);
-                Expression memberInit = Expression.MemberInit(ctor, val.Value);
+                var instance = scope.Instance.Peek();
+                Expression baseExpression;
+                if (val.Value.Count == 0)
+                {
+                    // No field has been selected for this type
+                    // -> Since we need to select something, select all(!) fields
+                    baseExpression = instance;
+                }
+                else
+                {
+                    // One or more fields have been selected for this type
+                    // -> Select only the requested fields.
+                    var ctor = Expression.New(val.Key);
+                    baseExpression = Expression.MemberInit(ctor, val.Value);
+                }
 
                 lastValue = Expression.Condition(
-                    Expression.TypeIs(scope.Instance.Peek(), val.Key),
-                    Expression.Convert(memberInit, scope.RuntimeType),
+                    Expression.TypeIs(instance, val.Key),
+                    Expression.Convert(baseExpression, scope.RuntimeType),
                     lastValue);
             }
 

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

@@ -214,6 +214,61 @@ await Snapshot
             .MatchAsync();
     }
 
+    [Fact]
+    public async Task Create_Union_Typename_Only()
+    {
+        // arrange
+        var tester =
+            _cache.CreateSchema(s_barEntities, OnModelCreating, configure: ConfigureSchema);
+
+        // act
+        var res1 = await tester.ExecuteAsync(
+            OperationRequestBuilder.New()
+                .SetDocument(
+                    @"
+                        {
+                            root {
+                                __typename
+                            }
+                        }")
+                .Build());
+
+        // assert
+        await Snapshot
+            .Create()
+            .AddResult(res1)
+            .MatchAsync();
+    }
+
+    [Fact]
+    public async Task Create_Union_Typename_Concrete_Mixed()
+    {
+        // arrange
+        var tester =
+            _cache.CreateSchema(s_barEntities, OnModelCreating, configure: ConfigureSchema);
+
+        // act
+        var res1 = await tester.ExecuteAsync(
+            OperationRequestBuilder.New()
+                .SetDocument(
+                    @"
+                        {
+                            root {
+                                __typename
+                                ... on Foo {
+                                    fooProp
+                                }
+                            }
+                        }")
+                .Build());
+
+        // assert
+        await Snapshot
+            .Create()
+            .AddResult(res1)
+            .MatchAsync();
+    }
+
     private static void OnModelCreating(ModelBuilder modelBuilder)
     {
         modelBuilder.Entity<AbstractType>()

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

@@ -0,0 +1,22 @@
+Result:
+---------------
+{
+  "data": {
+    "root": [
+      {
+        "__typename": "Bar"
+      },
+      {
+        "__typename": "Foo",
+        "fooProp": "FooProp"
+      }
+    ]
+  }
+}
+---------------
+
+SQL:
+---------------
+SELECT "d"."d" = 'bar', "d"."Id", "d"."Name", "d"."d", "d"."BarProp", "d"."FooProp", "d"."d" = 'foo'
+FROM "Data" AS "d"
+---------------

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

@@ -0,0 +1,21 @@
+Result:
+---------------
+{
+  "data": {
+    "root": [
+      {
+        "__typename": "Bar"
+      },
+      {
+        "__typename": "Foo"
+      }
+    ]
+  }
+}
+---------------
+
+SQL:
+---------------
+SELECT "d"."d" = 'bar', "d"."Id", "d"."Name", "d"."d", "d"."BarProp", "d"."FooProp", "d"."d" = 'foo'
+FROM "Data" AS "d"
+---------------