[ntuple] Handle processors created from bare models

tree/ntuple/v7/src/RNTupleProcessor.cxx

@@ -71,14 +71,19 @@ ROOT::Experimental::Internal::RNTupleProcessor::RNTupleProcessor(const std::vect
    model->Freeze();
    fEntry = model->CreateEntry();
 
-   // Use the value pointers from the model's in the entry managed by the processor. This way, the pointers returned by
-   // RNTupleModel::MakeField can be used in the processor loop to access the corresponding field values.
-   for (const auto &value : model->GetDefaultEntry()) {
+   for (const auto &value : *fEntry) {
       auto &field = value.GetField();
       auto token = fEntry->GetToken(field.GetFieldName());
-      auto valuePtr = value.GetPtr<void>();
-      fEntry->BindValue(token, valuePtr);
-      fFieldContexts.emplace_back(field.Clone(field.GetFieldName()), fEntry->GetToken(field.GetFieldName()));
+
+      // If the model has a default entry, use the value pointers from the entry in the entry managed by the
+      // processor. This way, the pointers returned by RNTupleModel::MakeField can be used in the processor loop to
+      // access the corresponding field values.
+      if (!model->IsBare()) {
+         auto valuePtr = model->GetDefaultEntry().GetPtr<void>(token);
+         fEntry->BindValue(token, valuePtr);
+      }
+
+      fFieldContexts.emplace_back(field.Clone(field.GetFieldName()), token);
    }
 
    ConnectFields();

tree/ntuple/v7/test/ntuple_processor.cxx

@@ -40,7 +40,7 @@ TEST(RNTupleProcessor, Basic)
    EXPECT_EQ(nEntries, 10);
 }
 
-TEST(RNTupleProcessor, BasicWithModel)
+TEST(RNTupleProcessor, WithModel)
 {
    FileRaii fileGuard("test_ntuple_processor_basic_with_model.root");
    {
@@ -68,6 +68,33 @@ TEST(RNTupleProcessor, BasicWithModel)
    }
 }
 
+TEST(RNTupleProcessor, WithBareModel)
+{
+   FileRaii fileGuard("test_ntuple_processor_basic_with_model.root");
+   {
+      auto model = RNTupleModel::Create();
+      auto fldX = model->MakeField<float>("x");
+      auto fldY = model->MakeField<float>("y");
+      auto fldZ = model->MakeField<float>("z");
+      auto ntuple = RNTupleWriter::Recreate(std::move(model), "ntuple", fileGuard.GetPath());
+
+      for (unsigned i = 0; i < 10; ++i) {
+         *fldX = static_cast<float>(i);
+         *fldY = static_cast<float>(i) * 2.f;
+         *fldZ = static_cast<float>(i) * 3.f;
+         ntuple->Fill();
+      }
+   }
+
+   auto model = RNTupleModel::CreateBare();
+   model->MakeField<float>("y");
+   std::vector<RNTupleSourceSpec> ntuples = {{"ntuple", fileGuard.GetPath()}};
+
+   for (const auto &entry : RNTupleProcessor(ntuples, std::move(model))) {
+      EXPECT_EQ(static_cast<float>(entry.GetGlobalEntryIndex()) * 2.f, *entry->GetPtr<float>("y"));
+   }
+}
+
 TEST(RNTupleProcessor, SimpleChain)
 {
    FileRaii fileGuard1("test_ntuple_processor_simple_chain1.root");