Refactored Query Planner to allow top queries.

src/HotChocolate/Fusion/src/Core/Execution/Nodes/QueryPlan.cs

@@ -174,7 +174,7 @@ public async Task<IQueryResult> ExecuteAsync(
             operationContext.Result.SetExtension(QueryPlanHashProp, _hash);
         }
 
-        // we store the context on the result for unit tests.
+        // we store the context on the result for unit tests.x
         operationContext.Result.SetContextData(QueryPlanProp, context.QueryPlan);
 
         // Enqueue root node to initiate the execution process.

src/HotChocolate/Fusion/src/Core/Execution/Nodes/Resolve.cs

@@ -42,7 +42,7 @@ protected override async Task OnExecuteAsync(
     {
         if (state.TryGetState(SelectionSet, out var executionState))
         {
-            var requests =  new SubgraphGraphQLRequest[executionState.Count];
+            var requests = new SubgraphGraphQLRequest[executionState.Count];
 
             // first we will create request for all of our selection sets.
             InitializeRequests(context, executionState, requests);
@@ -57,7 +57,12 @@ protected override async Task OnExecuteAsync(
             // but need to wait until the transport layer is finished and disposes the result.
             context.Result.RegisterForCleanup(responses, ReturnResults);
 
-            ProcessResponses(context, executionState, requests, responses, SubgraphName);
+            // we need to lock the state before mutating it since there could be multiple
+            // query plan nodes be interested in it.
+            lock (executionState)
+            {
+                ProcessResponses(context, executionState, requests, responses, SubgraphName);
+            }
         }
     }
 

src/HotChocolate/Fusion/src/Core/Execution/Nodes/ResolveByKeyBatch.cs

@@ -75,7 +75,12 @@ protected override async Task OnExecuteAsync(
             // for cleanup so that the memory can be released at the end of the execution.
             context.Result.RegisterForCleanup(response, ReturnResult);
 
-            ProcessResult(context, response, batchExecutionState);
+            // we need to lock the state before mutating it since there could be multiple
+            // query plan nodes be interested in it.
+            lock (executionState)
+            {
+                ProcessResult(context, response, batchExecutionState);
+            }
         }
     }
 

src/HotChocolate/Fusion/src/Core/Planning/Pipeline/ExecutionNodeBuilderMiddleware.cs

@@ -295,10 +295,19 @@ private Subscribe CreateSubscribeNode(
 
     private ISelectionSet ResolveSelectionSet(
         QueryPlanContext context,
-        ExecutionStep executionStep)
-        => executionStep.ParentSelection is null
+        SelectionExecutionStep executionStep)
+    {
+        if (executionStep.Resolver is null &&
+            executionStep.SelectionResolvers.Count == 0 &&
+            executionStep.ParentSelectionPath is not null)
+        {
+            return context.Operation.RootSelectionSet;
+        }
+
+        return executionStep.ParentSelection is null
             ? context.Operation.RootSelectionSet
             : context.Operation.GetSelectionSet(
                 executionStep.ParentSelection,
                 _schema.GetType<ObjectType>(executionStep.SelectionSetTypeMetadata.Name));
+    }
 }

src/HotChocolate/Fusion/src/Core/Planning/Pipeline/ExecutionStepDiscoveryMiddleware.cs

@@ -60,6 +60,7 @@ public void Invoke(QueryPlanContext context, QueryPlanDelegate next)
             selectionSetType,
             selections,
             null,
+            null,
             false);
 
         while (backlog.TryDequeue(out var item))
@@ -70,6 +71,7 @@ public void Invoke(QueryPlanContext context, QueryPlanDelegate next)
                 item.DeclaringTypeMetadata,
                 item.Selections,
                 item.ParentSelection,
+                item.SelectionPath,
                 item.PreferBatching);
         }
 
@@ -82,12 +84,14 @@ private void CreateExecutionSteps(
         ObjectTypeMetadata selectionSetTypeMetadata,
         IReadOnlyList<ISelection> selections,
         ISelection? parentSelection,
+        SelectionPath? parentSelectionPath,
         bool preferBatching)
     {
         var variablesInContext = new HashSet<string>();
         var operation = context.Operation;
         List<ISelection>? leftovers = null;
-
+        var path = new List<ISelection>();
+
         // if this is the root selection set of a query we will
         // look for some special selections.
         if (!context.HasHandledSpecialQueryFields && parentSelection is null)
@@ -117,6 +121,7 @@ private void CreateExecutionSteps(
                 context.NextStepId(),
                 subgraph,
                 parentSelection,
+                parentSelectionPath,
                 _schema.GetType<IObjectType>(selectionSetTypeMetadata.Name),
                 selectionSetTypeMetadata);
             leftovers = null;
@@ -134,6 +139,9 @@ private void CreateExecutionSteps(
 
             foreach (var selection in current)
             {
+                var pathIndex = path.Count;
+                path.Add(selection);
+
                 var field = selection.Field;
                 var fieldInfo = selectionSetTypeMetadata.Fields[field.Name];
 
@@ -191,10 +199,29 @@ private void CreateExecutionSteps(
                         backlog,
                         operation,
                         selection,
+                        parentSelectionPath,
+                        path,
                         executionStep,
                         preferBatching,
                         context.ParentSelections);
                 }
+
+                path.RemoveAt(pathIndex);
+            }
+
+            // if the current execution step has now way to resolve the data
+            // we will try to resolve it from the root.
+            if(executionStep.ParentSelection is not null && 
+                executionStep.ParentSelectionPath is not null &&
+                executionStep.Resolver is null && 
+                executionStep.SelectionResolvers.Count == 0)
+            {
+                if (!EnsureStepCanBeResolvedFromRoot(
+                    executionStep.SubgraphName,
+                    executionStep.ParentSelectionPath))
+                {
+                    throw ThrowHelper.NoResolverInContext();
+                }
             }
 
             context.Steps.Add(executionStep);
@@ -258,6 +285,8 @@ private void CollectNestedSelections(
         Queue<BacklogItem> backlog,
         IOperation operation,
         ISelection parentSelection,
+        SelectionPath? rootSelectionPath,
+        List<ISelection> path,
         SelectionExecutionStep executionStep,
         bool preferBatching,
         Dictionary<ISelection, ISelection> parentSelectionLookup)
@@ -273,6 +302,8 @@ private void CollectNestedSelections(
                 backlog,
                 operation,
                 parentSelection,
+                rootSelectionPath,
+                path,
                 executionStep,
                 possibleType,
                 preferBatching,
@@ -284,6 +315,8 @@ private void CollectNestedSelections(
         Queue<BacklogItem> backlog,
         IOperation operation,
         ISelection parentSelection,
+        SelectionPath? rootSelectionPath,
+        List<ISelection> path,
         SelectionExecutionStep executionStep,
         IObjectType possibleType,
         bool preferBatching,
@@ -297,6 +330,9 @@ private void CollectNestedSelections(
 
         foreach (var selection in selectionSet.Selections)
         {
+            var pathIndex = path.Count;
+            path.Add(selection);
+
             parentSelectionLookup.TryAdd(selection, parentSelection);
             var field = declaringType.Fields[selection.Field.Name];
 
@@ -346,6 +382,8 @@ private void CollectNestedSelections(
                         backlog,
                         operation,
                         selection,
+                        rootSelectionPath,
+                        path,
                         executionStep,
                         preferBatching,
                         parentSelectionLookup);
@@ -355,19 +393,36 @@ private void CollectNestedSelections(
             {
                 (leftovers ??= new()).Add(selection);
             }
+
+            path.RemoveAt(pathIndex);
         }
 
         if (leftovers is not null)
         {
             backlog.Enqueue(
                 new BacklogItem(
                     parentSelection,
+                    CreateSelectionPath(rootSelectionPath, path),
                     declaringType,
                     leftovers,
                     preferBatching));
         }
     }
 
+    private static SelectionPath? CreateSelectionPath(SelectionPath? rootPath, List<ISelection> pathSegments)
+    {
+        var parent = rootPath;
+
+        for (var i = 0; i < pathSegments.Count; i++)
+        {
+            parent = parent is null
+                ? new SelectionPath(pathSegments[i])
+                : parent.Append(pathSegments[i]);
+        }
+
+        return parent;
+    }
+
     private static void AddIntrospectionStepIfNotExists(
         QueryPlanContext context,
         IObjectField field,
@@ -497,6 +552,8 @@ private SelectionExecutionStep CreateNodeNestedExecutionSteps(
             backlog,
             operation,
             nodeSelection,
+            null,
+            new List<ISelection>(),
             executionStep,
             entityType,
             preferBatching,
@@ -733,14 +790,38 @@ private static bool IsNodeField(IObjectField field, IOperation operation)
             field.DeclaringType.Equals(operation.RootType) &&
             (field.Name.EqualsOrdinal("node") || field.Name.EqualsOrdinal("nodes"));
 
+    private bool EnsureStepCanBeResolvedFromRoot(
+        string subgraphName,
+        SelectionPath path)
+    {
+        var current = path;
+
+        while (current is not null)
+        {
+            var typeMetadata = _config.GetType<ObjectTypeMetadata>(path.Selection.DeclaringType.Name);
+
+            if (!typeMetadata.Fields[path.Selection.Field.Name].Bindings.ContainsSubgraph(subgraphName))
+            {
+                return false;
+            }
+
+            current = current.Parent;
+        }
+
+        return true;
+    }
+
     private readonly struct BacklogItem(
         ISelection parentSelection,
+        SelectionPath? selectionPath,
         ObjectTypeMetadata declaringTypeMetadata,
         IReadOnlyList<ISelection> selections,
         bool preferBatching)
     {
         public ISelection ParentSelection { get; } = parentSelection;
 
+        public SelectionPath? SelectionPath { get; } = selectionPath;
+
         public ObjectTypeMetadata DeclaringTypeMetadata { get; } = declaringTypeMetadata;
 
         public IReadOnlyList<ISelection> Selections { get; } = selections;

src/HotChocolate/Fusion/src/Core/Planning/Pipeline/ExecutionTreeBuilderMiddleware.cs

@@ -251,11 +251,21 @@ private static void TryEnqueueBatch(
     private ISelectionSet ResolveSelectionSet(
         QueryPlanContext context,
         ExecutionStep executionStep)
-        => executionStep.ParentSelection is null
+    {
+        if (executionStep is SelectionExecutionStep selectionExecStep && 
+            selectionExecStep.Resolver is null &&
+            selectionExecStep.SelectionResolvers.Count == 0 &&
+            selectionExecStep.ParentSelectionPath is not null)
+        {
+            return context.Operation.RootSelectionSet;
+        }
+
+        return executionStep.ParentSelection is null
             ? context.Operation.RootSelectionSet
             : context.Operation.GetSelectionSet(
                 executionStep.ParentSelection,
                 _schema.GetType<Types.ObjectType>(executionStep.SelectionSetTypeMetadata.Name));
+    }
 
     private readonly record struct BacklogItem(NodeAndStep[] Batch, Sequence Parent);
 }

src/HotChocolate/Fusion/src/Core/Planning/Pipeline/FieldRequirementsPlannerMiddleware.cs

@@ -196,6 +196,7 @@ private static void RegisterRequirementStep(
             context.NextStepId(),
             subgraph,
             parentSelection,
+            null,
             selection.DeclaringType,
             typeMetadata);
 

src/HotChocolate/Fusion/src/Core/Planning/RequestFormatters/RequestDocumentFormatter.cs

@@ -59,6 +59,15 @@ internal RequestDocument CreateRequestDocument(
             path = p;
         }
 
+        if (executionStep.Resolver is null &&
+            executionStep.SelectionResolvers.Count == 0 &&
+            executionStep.ParentSelectionPath is not null)
+        {
+            rootSelectionSetNode = CreateRootLevelQuery(
+                executionStep.ParentSelectionPath,
+                rootSelectionSetNode);
+        }
+
         var operationDefinitionNode = new OperationDefinitionNode(
             null,
             context.CreateRemoteOperationName(),
@@ -75,6 +84,26 @@ internal RequestDocument CreateRequestDocument(
             path);
     }
 
+    private SelectionSetNode CreateRootLevelQuery(
+        SelectionPath path, 
+        SelectionSetNode selectionSet)
+    {
+        var current = path;
+
+        while (current is not null)
+        {
+            selectionSet = new SelectionSetNode(
+                new[]
+                {
+                    current.Selection.SyntaxNode.WithSelectionSet(selectionSet)
+                });
+
+            current = current.Parent;
+        }
+
+        return selectionSet;
+    }
+
     protected virtual SelectionSetNode CreateRootSelectionSetNode(
         QueryPlanContext context,
         SelectionExecutionStep executionStep)

src/HotChocolate/Fusion/src/Core/Planning/Steps/SelectionExecutionStep.cs

@@ -33,7 +33,7 @@ public SelectionExecutionStep(
         string subgraphName,
         IObjectType selectionSet,
         ObjectTypeMetadata selectionSetTypeMetadata)
-        : this(id, subgraphName, null, selectionSet, selectionSetTypeMetadata)
+        : this(id, subgraphName, null, null, selectionSet, selectionSetTypeMetadata)
     {
     }
 
@@ -49,6 +49,9 @@ public SelectionExecutionStep(
     /// <param name="parentSelection">
     /// The parent selection of this execution step.
     /// </param>
+    /// <param name="parentSelectionPath">
+    /// The selection path from which this execution step was spawned.
+    /// </param>
     /// <param name="selectionSet">
     /// The selection set that is part of this execution step.
     /// </param>
@@ -59,17 +62,24 @@ public SelectionExecutionStep(
         int id,
         string subgraphName,
         ISelection? parentSelection,
+        SelectionPath? parentSelectionPath,
         IObjectType selectionSet,
         ObjectTypeMetadata selectionSetTypeMetadata)
         : base(id, parentSelection, selectionSet, selectionSetTypeMetadata)
     {
         SubgraphName = subgraphName;
+        ParentSelectionPath = parentSelectionPath;
     }
 
     /// <summary>
     /// Gets the subgraph from which this execution step will fetch data.
     /// </summary>
     public string SubgraphName { get; }
+
+    /// <summary>
+    /// Gets the selection path from which this execution step was spawned.
+    /// </summary>
+    public SelectionPath? ParentSelectionPath { get; }
 
     /// <summary>
     /// Gets the resolver for this execution step.