Resolve issue where an Entity is required in subgraphs and remove non-resolvable from _entity output

src/HotChocolate/ApolloFederation/src/ApolloFederation/FederationTypeInterceptor.cs

@@ -12,8 +12,8 @@
 using HotChocolate.Types.Descriptors;
 using HotChocolate.Types.Descriptors.Definitions;
 using HotChocolate.Types.Helpers;
-using static HotChocolate.ApolloFederation.ThrowHelper;
 using static HotChocolate.ApolloFederation.FederationContextData;
+using static HotChocolate.ApolloFederation.ThrowHelper;
 using static HotChocolate.Types.TagHelper;
 
 namespace HotChocolate.ApolloFederation;
@@ -95,7 +95,10 @@ public override IEnumerable<TypeReference> RegisterMoreTypes(
 
         _registeredTypes = true;
         yield return _typeInspector.GetTypeRef(typeof(_Service));
-        yield return _typeInspector.GetTypeRef(typeof(_EntityType));
+        if (_entityTypes.Count > 0)
+        {
+            yield return _typeInspector.GetTypeRef(typeof(_EntityType));
+        }
         yield return _typeInspector.GetTypeRef(typeof(_AnyType));
         yield return _typeInspector.GetTypeRef(typeof(FieldSetType));
 
@@ -283,14 +286,6 @@ internal override void OnAfterResolveRootType(
         }
     }
 
-    public override void OnTypesInitialized()
-    {
-        if (_entityTypes.Count == 0)
-        {
-            throw EntityType_NoEntities();
-        }
-    }
-
     public override void OnBeforeCompleteType(
         ITypeCompletionContext completionContext,
         DefinitionBase definition)
@@ -388,7 +383,10 @@ private void AddServiceTypeToQueryType(
 
         var objectTypeDefinition = (ObjectTypeDefinition)definition!;
         objectTypeDefinition.Fields.Add(ServerFields.CreateServiceField(_context));
-        objectTypeDefinition.Fields.Add(ServerFields.CreateEntitiesField(_context));
+        if (_entityTypes.Count > 0)
+        {
+            objectTypeDefinition.Fields.Add(ServerFields.CreateEntitiesField(_context));
+        }
     }
 
     private void ApplyMethodLevelReferenceResolvers(
@@ -432,8 +430,15 @@ private void AddToUnionIfHasTypeLevelKeyDirective(
         ObjectType objectType,
         ObjectTypeDefinition objectTypeDefinition)
     {
-        if (objectTypeDefinition.Directives.Any(d => d.Value is KeyDirective) ||
-            objectTypeDefinition.Fields.Any(f => f.ContextData.ContainsKey(KeyMarker)))
+        if (objectTypeDefinition.Directives.FirstOrDefault(d => d.Value is KeyDirective) is { } keyDirective &&
+            ((KeyDirective)keyDirective.Value).Resolvable)
+        {
+            _entityTypes.Add(objectType);
+            return;
+        }
+
+        if (objectTypeDefinition.Fields.Any(f => f.ContextData.TryGetValue(KeyMarker, out var resolvable) &&
+                resolvable is true))
         {
             _entityTypes.Add(objectType);
         }
@@ -457,11 +462,22 @@ private void AggregatePropertyLevelKeyDirectives(
 
         IReadOnlyList<ObjectFieldDefinition> fields = objectTypeDefinition.Fields;
         var fieldSet = new StringBuilder();
+        bool? resolvable = null;
 
         foreach (var fieldDefinition in fields)
         {
-            if (fieldDefinition.ContextData.ContainsKey(KeyMarker))
+            if (fieldDefinition.ContextData.TryGetValue(KeyMarker, out var value) &&
+                value is bool currentResolvable)
             {
+                if (resolvable is null)
+                {
+                    resolvable = currentResolvable;
+                }
+                else if (resolvable != currentResolvable)
+                {
+                    throw Key_FieldSet_ResolvableMustBeConsistent(fieldDefinition.Member!);
+                }
+
                 if (fieldSet.Length > 0)
                 {
                     fieldSet.Append(' ');
@@ -472,7 +488,7 @@ private void AggregatePropertyLevelKeyDirectives(
         }
 
         // add the key directive with the dynamically generated field set.
-        AddKeyDirective(objectTypeDefinition, fieldSet.ToString());
+        AddKeyDirective(objectTypeDefinition, fieldSet.ToString(), resolvable ?? true);
 
         // register dependency to the key directive so that it is completed before
         // we complete this type.
@@ -486,9 +502,12 @@ private void AggregatePropertyLevelKeyDirectives(
             discoveryContext.Dependencies.Add(new(directiveDefinition.Type));
         }
 
-        // since this type has now a key directive we also need to add this type to
-        // the _Entity union type.
-        _entityTypes.Add(objectType);
+        if (resolvable ?? true)
+        {
+            // since this type has now a key directive we also need to add this type to
+            // the _Entity union type provided that the key is resolvable.
+            _entityTypes.Add(objectType);
+        }
     }
 
     private void AddMemberTypesToTheEntityUnionType(
@@ -507,11 +526,12 @@ private void AddMemberTypesToTheEntityUnionType(
 
     private void AddKeyDirective(
         ObjectTypeDefinition objectTypeDefinition,
-        string fieldSet)
+        string fieldSet,
+        bool resolvable)
     {
         objectTypeDefinition.Directives.Add(
             new DirectiveDefinition(
-                new KeyDirective(fieldSet),
+                new KeyDirective(fieldSet, resolvable),
                 _keyDirectiveReference));
     }
-}
+}

src/HotChocolate/ApolloFederation/src/ApolloFederation/Properties/FederationResources.resx

@@ -171,9 +171,6 @@
   <data name="FieldDescriptorExtensions_Override_From_CannotBeNullOrEmpty" xml:space="preserve">
     <value>Value cannot be null or empty.</value>
   </data>
-  <data name="ThrowHelper_EntityType_NoEntities" xml:space="preserve">
-    <value>The schema has no types with a KeyDirective and therefore no entities. Apollo federation requires at least one entity.</value>
-  </data>
   <data name="ThrowHelper_EntityResolver_NoEntityResolverFound" xml:space="preserve">
     <value>The apollo gateway tries to resolve an entity for which no EntityResolver method was found.</value>
   </data>

src/HotChocolate/ApolloFederation/src/ApolloFederation/ThrowHelper.cs

@@ -83,18 +83,6 @@ public static SerializationException Scalar_CannotParseValue(
                 .Build(),
             scalarType);
 
-    /// <summary>
-    /// The schema doesn't contain any types with a key directive
-    /// and therefore no entities. An Apollo federation service
-    /// needs at least one entity.
-    /// </summary>
-    public static SchemaException EntityType_NoEntities() =>
-        new SchemaException(
-            SchemaErrorBuilder.New()
-                .SetMessage(ThrowHelper_EntityType_NoEntities)
-                // .SetCode(ErrorCodes.ApolloFederation.NoEntitiesDeclared)
-                .Build());
-
     /// <summary>
     /// The apollo gateway tries to resolve an entity for which no
     /// EntityResolver method was found.
@@ -120,13 +108,13 @@ public static SchemaException Key_FieldSet_CannotBeEmpty(
                 .Build());
 
     /// <summary>
-    /// The key attribute is used on the type level without specifying the fieldset.
+    /// The key attribute is used on the field level whilst specifying the fieldset.
     /// </summary>
     public static SchemaException Key_FieldSet_MustBeEmpty(
         MemberInfo member)
     {
         var type = member.ReflectedType ?? member.DeclaringType!;
-        
+
         return new SchemaException(
             SchemaErrorBuilder.New()
                 .SetMessage("The specified key attribute must not specify a fieldset when annotated to a field.")
@@ -136,6 +124,23 @@ public static SchemaException Key_FieldSet_MustBeEmpty(
                 .Build());
     }
 
+    /// <summary>
+    /// The key attribute is used on the field level with inconsistent resolvable values.
+    /// </summary>
+    public static SchemaException Key_FieldSet_ResolvableMustBeConsistent(
+        MemberInfo member)
+    {
+        var type = member.ReflectedType ?? member.DeclaringType!;
+
+        return new SchemaException(
+            SchemaErrorBuilder.New()
+                .SetMessage("The specified key attributes must shared the same resolvable values when annotated on multiple fields.")
+                .SetExtension("type", type.FullName ?? type.Name)
+                .SetExtension("member", member.Name)
+                // .SetCode(ErrorCodes.ApolloFederation.KeyFieldSetNullOrEmpty)
+                .Build());
+    }
+
     /// <summary>
     /// The provides attribute is used and the fieldset is set to <c>null</c> or
     /// <see cref="string.Empty"/>.
@@ -209,10 +214,10 @@ public static SchemaException FederationVersion_Unknown(
                     ThrowHelper_FederationVersion_Unknown,
                     version)
                 .Build());
-    
+
     public static SchemaException Contact_Not_Repeatable() =>
         new SchemaException(
             SchemaErrorBuilder.New()
                 .SetMessage("The @contact directive is not repeatable and can.")
-                .Build()); 
-}
+                .Build());
+}

src/HotChocolate/ApolloFederation/src/ApolloFederation/Types/Directives/KeyAttribute.cs

@@ -127,11 +127,11 @@ private void ConfigureField(MemberInfo member, IDescriptor descriptor)
         switch (descriptor)
         {
             case IObjectFieldDescriptor fieldDesc:
-                fieldDesc.Extend().Definition.ContextData.TryAdd(KeyMarker, true);
+                fieldDesc.Extend().Definition.ContextData.TryAdd(KeyMarker, Resolvable);
                 break;
 
             case IInterfaceFieldDescriptor fieldDesc:
-                fieldDesc.Extend().Definition.ContextData.TryAdd(KeyMarker, true);
+                fieldDesc.Extend().Definition.ContextData.TryAdd(KeyMarker, Resolvable);
                 break;
         }
     }

src/HotChocolate/ApolloFederation/src/ApolloFederation/Types/Directives/KeyDescriptorExtensions.cs

@@ -47,7 +47,7 @@ public static IEntityResolverDescriptor Key(
     {
         ArgumentNullException.ThrowIfNull(descriptor);
         ArgumentException.ThrowIfNullOrEmpty(fieldSet);
-        
+
         descriptor.Directive(new KeyDirective(fieldSet, resolvable));
         return new EntityResolverDescriptor<object>(descriptor);
     }
@@ -139,7 +139,7 @@ public static IInterfaceTypeDescriptor Key(
     {
         ArgumentNullException.ThrowIfNull(descriptor);
         ArgumentException.ThrowIfNullOrEmpty(fieldSet);
-        
+
         return descriptor.Directive(new KeyDirective(fieldSet, resolvable));
     }
-}
+}