From 2f13e4413c81045b6a2d926c7dd6f1e480c5dbc2 Mon Sep 17 00:00:00 2001
From: silverweed <giacomo.parolini@cern.ch>
Date: Thu, 30 Oct 2025 10:29:40 +0100
Subject: [PATCH] [ntuple] Merger: add field version check to schema validation

---
 tree/ntuple/src/RNTupleMerger.cxx | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/tree/ntuple/src/RNTupleMerger.cxx b/tree/ntuple/src/RNTupleMerger.cxx
index 9b5462fbf2446..afba0635654e3 100644
--- a/tree/ntuple/src/RNTupleMerger.cxx
+++ b/tree/ntuple/src/RNTupleMerger.cxx
@@ -488,6 +488,17 @@ CompareDescriptorStructure(const ROOT::RNTupleDescriptor &dst, const ROOT::RNTup
          errors.push_back(ss.str());
       }
 
+      // Require that field versions match
+      const auto srcFldVer = field.fSrc->GetFieldVersion();
+      const auto dstFldVer = field.fDst->GetFieldVersion();
+      if (srcFldVer != dstFldVer) {
+         std::stringstream ss;
+         ss << "Field `" << field.fSrc->GetFieldName()
+            << "` has a different field version than previously-seen field with the same name (old: " << dstFldVer
+            << ", new: " << srcFldVer << ")";
+         errors.push_back(ss.str());
+      }
+
       const auto srcRole = field.fSrc->GetStructure();
       const auto dstRole = field.fDst->GetStructure();
       if (srcRole != dstRole) {