From 02bcb56eb402606fab0823442bee99f21a2ea92a Mon Sep 17 00:00:00 2001
From: tmadlener <thomas.madlener@desy.de>
Date: Thu, 3 Sep 2020 15:03:47 +0200
Subject: [PATCH] Fix handling of file switching for event meta data

---
 include/podio/ROOTReader.h |  3 +++
 src/ROOTReader.cc          | 14 ++++++++++----
 2 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/include/podio/ROOTReader.h b/include/podio/ROOTReader.h
index b6dd335e1..1e4d48214 100755
--- a/include/podio/ROOTReader.h
+++ b/include/podio/ROOTReader.h
@@ -6,6 +6,7 @@
 #include <string>
 #include <vector>
 #include <iostream>
+#include <utility>
 
 // forward declarations
 class TClass;
@@ -76,6 +77,8 @@ class ROOTReader : public IReader {
     std::map<int,GenericParameters>* readRunMetaData() override final ;
 
   private:
+    std::pair<TTree*, unsigned> getLocalTreeAndEntry(const std::string& treename);
+
     typedef std::pair<CollectionBase*, std::string> Input;
     std::vector<Input> m_inputs;
     std::map<std::string, std::pair<TClass*,TClass*> > m_storedClasses;
diff --git a/src/ROOTReader.cc b/src/ROOTReader.cc
index fb4e786e0..67bce70a4 100755
--- a/src/ROOTReader.cc
+++ b/src/ROOTReader.cc
@@ -13,23 +13,29 @@
 
 namespace podio {
 
+  std::pair<TTree*, unsigned> ROOTReader::getLocalTreeAndEntry(const std::string& treename) {
+    auto localEntry = m_chain->LoadTree(m_eventNumber);
+    auto* tree = static_cast<TTree*>(m_chain->GetFile()->Get(treename.c_str()));
+    return {tree, localEntry};
+  }
+
   GenericParameters* ROOTReader::readEventMetaData(){
     GenericParameters* emd = new GenericParameters() ;
-    auto evt_metadatatree = static_cast<TTree*>(m_chain->GetFile()->Get("evt_metadata"));
+    auto [evt_metadatatree, entry] = getLocalTreeAndEntry("evt_metadata");
     evt_metadatatree->SetBranchAddress("evtMD",&emd);
-    evt_metadatatree->GetEntry(m_eventNumber);
+    evt_metadatatree->GetEntry(entry);
     return emd ;
   }
   std::map<int,GenericParameters>* ROOTReader::readCollectionMetaData(){
     auto* emd = new std::map<int,GenericParameters> ;
-    auto col_metadatatree = static_cast<TTree*>(m_chain->GetFile()->Get("col_metadata"));
+    auto [col_metadatatree, dummy] = getLocalTreeAndEntry("col_metadata");
     col_metadatatree->SetBranchAddress("colMD",&emd);
     col_metadatatree->GetEntry(0);
     return emd ;
   }
   std::map<int,GenericParameters>* ROOTReader::readRunMetaData(){
     auto* emd = new std::map<int,GenericParameters> ;
-    auto run_metadatatree = static_cast<TTree*>(m_chain->GetFile()->Get("run_metadata"));
+    auto [run_metadatatree, dummy] = getLocalTreeAndEntry("run_metadata");
     run_metadatatree->SetBranchAddress("runMD",&emd);
     run_metadatatree->GetEntry(0);
     return emd ;