Cannot access subfields through RNTupleProcessor

[enirolf]

Check duplicate issues.

• Checked for duplicates

Description

Subfields that have been registered through RNTupleModel::RegisterModel cannot be accessed through the RNTupleProcessor when it has been created with such a model. To enable this, the subfield's RFieldContext will need to be somehow linked to that of its parent, so its fieldId can be properly retrieved in the processor's ConnectField method.

Reproducer

using ROOT::Experimental::RFieldBase;
using ROOT::Experimental::RNTupleModel;
using ROOT::Experimental::RNTupleOpenSpec;
using ROOT::Experimental::RNTupleProcessor;
using ROOT::Experimental::RNTupleWriter;

struct MyStruct {
  float a;
};

void ntuple_proc_subfields() {
  {
    auto model = RNTupleModel::Create();
    auto myStruct = model->MakeField<MyStruct>("struct");
    auto ntuple = RNTupleWriter::Recreate(std::move(model), "ntuple",
                                          "ntuple_proc_subfields1.root");
    *myStruct = MyStruct{1.f};
    ntuple->Fill();
  }
  {
    auto model = RNTupleModel::Create();
    auto myStruct = model->MakeField<MyStruct>("struct", MyStruct{2.f});
    auto ntuple = RNTupleWriter::Recreate(std::move(model), "ntuple",
                                          "ntuple_proc_subfields2.root");
    *myStruct = MyStruct{2.f};
    ntuple->Fill();
  }

  auto model = RNTupleModel::Create();
  auto structField = RFieldBase::Create("struct", "MyStruct").Unwrap();
  model->AddField(std::move(structField));
  model->RegisterSubfield("struct.a");

  std::vector<RNTupleOpenSpec> ntuples = {{"ntuple", "ntuple_proc_subfields1.root"},
                                          {"ntuple", "ntuple_proc_subfields2.root"}};

  auto proc = RNTupleProcessor::CreateChain(ntuples, std::move(model));
  for (const auto &entry : *proc) {
    std::cout << entry.GetPtr<float>("struct.a") << std::endl;
  }
}

Currently this throws an RException with the message field "a" not found in current RNTuple. It should run and print the values instead.

ROOT version

any

Installation method

any

Operating system

any

Additional context

No response