Script to create multiple trees split on different files.

createMultipleTrees.py

import ROOT
import sys

# A simple helper function to fill a test tree: this makes the example stand-alone.
def fill_tree(treeName, fileName, b1, b2):
   opts = ROOT.ROOT.RDF.RSnapshotOptions();
   opts.fMode = "UPDATE";

   branchList = ROOT.vector('string')()
   for branchName in [b1, b2]:
     branchList.push_back(branchName)
     
   tdf = ROOT.ROOT.RDataFrame(200)
   tdf.Define(b1, "(double) tdfentry_")\
      .Define(b2, "(int) tdfentry_ * tdfentry_")\
      .Snapshot(treeName, fileName, branchList, opts)

# We prepare some trees split on different files
for f in range(20):
   filename = "sample{}.root".format(f)
   fill_tree("processInfoTree", filename, "luminosity", "cross_section")
   fill_tree("weightsTree",     filename, "weight", "weight1")
   fill_tree("randomTree",      filename, "x", "x2")
   fill_tree("anotherTree",     filename, "y", "y2")

MultipleChainTest.cpp

{
using namespace ROOT;

auto filenames = std::vector<std::string>();
for(int i = 0; i<20 ; i++){
   std::string s = "sample" + std::to_string(i) + ".root";
   filenames.push_back(s);
}

// process info tree
TChain *info_chain = new TChain("processInfoTree");
//  weight tree
TChain *weight_chain = new TChain("weightsTree");
//  weight 2 tree
TChain *weight2_chain = new TChain("randomTree");
//  weight 3 tree
TChain *weight3_chain = new TChain("anotherTree");

for(auto f: filenames){
  cout << f << endl;
  info_chain->Add(f.c_str());

  weight_chain->Add(f.c_str());

  weight2_chain->Add(f.c_str());

  weight3_chain->Add(f.c_str());
}

info_chain->AddFriend(weight_chain, "weights");
info_chain->AddFriend(weight2_chain, "weights");
info_chain->AddFriend(weight3_chain, "weights");

RDataFrame* rdf = new RDataFrame(*info_chain);

cout << "Branches: " << endl;
for(auto c : rdf->GetColumnNames())
   cout << c << endl;

cout << *rdf->Filter("cross_section > 0").Count() << endl;
cout << *rdf->Filter("weight1 > 0.5").Count() << endl; // Fails here

delete rdf;
delete info_chain;
delete weight_chain;
}

Run:

python createMultipleTrees.py

This creates 20 files named sampleX.root where X = [0...20). Then:

root MultipleChainTest.cpp

ROOT version taken from the LCG nightlies of Friday 19th:

source /cvmfs/sft.cern.ch/lcg/views/dev3/Fri/x86_64-centos7-gcc62-opt/setup.sh

Error:

$ root MultipleChainTest.cpp
root [0] 
Processing MultipleChainTest.cpp...
sample0.root
sample1.root
sample2.root
sample3.root
sample4.root
sample5.root
sample6.root
sample7.root
sample8.root
sample9.root
sample10.root
sample11.root
sample12.root
sample13.root
sample14.root
sample15.root
sample16.root
sample17.root
sample18.root
sample19.root
Branches: 
luminosity
cross_section
weights.weight
weight
weights.weight1
weight1
weights.x
x
weights.x2
x2
weights.y
y
weights.y2
y2
3980
Error in <TTreeReaderValueBase::CreateProxy()>: The tree does not have a branch called weight1. You could check with TTree::Print() for available branches.
0

Using different aliases for the friend trees I did not manage to reproduce the problem:

info_chain->AddFriend(weight_chain, "weights");
info_chain->AddFriend(weight2_chain, "weights2");
info_chain->AddFriend(weight3_chain, "weights3");