From 5eaaef20a025836b897cf16e902970e49f513941 Mon Sep 17 00:00:00 2001
From: darknight0847 <dev4447f@gmail.com>
Date: Sat, 8 Nov 2025 14:26:09 +0530
Subject: [PATCH] fix(user): Make skin and bone compatible with
 attachImplements ResolveReferences for mjCSkin and adds prefixingin
 mjCModel::operator+= to ensure skin bone body names arecorrectly updated when
 a model is attached.This prevents an 'unknown body' error during
 compilationof an attached model.Fixes #2923

---
 src/user/user_model.cc   | 14 ++++++++++++++
 src/user/user_objects.cc | 23 ++++++++++++++++++++++-
 2 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/src/user/user_model.cc b/src/user/user_model.cc
index ad7af6ddcf..75da5ed2df 100644
--- a/src/user/user_model.cc
+++ b/src/user/user_model.cc
@@ -447,7 +447,20 @@ mjCModel& mjCModel::operator+=(const mjCModel& other) {
     // do not copy assets for self-attach
     // TODO: asset should be copied only when referenced
     CopyList(meshes_, other.meshes_);
+    int old_nskin = skins_.size();
     CopyList(skins_, other.skins_);
+    int new_nskin = skins_.size();
+
+    // Iterate ONLY over the newly added skins
+    for (int i = old_nskin; i < new_nskin; ++i) {
+      mjCSkin* skin = skins_[i];
+      for (std::string& bodyname : skin->bodyname_) {
+        if (!bodyname.empty()) {
+          bodyname = prefix + bodyname;
+        }
+      }
+    }
+
     CopyList(hfields_, other.hfields_);
     CopyList(textures_, other.textures_);
     CopyList(materials_, other.materials_);
@@ -1921,6 +1934,7 @@ void mjCModel::IndexAssets(bool discard) {
         throw mjCError(skin, "material '%s' not found in skin %d", skin->material_.c_str(), i);
       }
     }
+    skin->ResolveReferences(this);
   }
 
   // materials referenced in sites
diff --git a/src/user/user_objects.cc b/src/user/user_objects.cc
index 758783e4d5..a582cf260f 100644
--- a/src/user/user_objects.cc
+++ b/src/user/user_objects.cc
@@ -2598,7 +2598,28 @@ void mjCBody::Compile(void) {
 }
 
 
-
+// skin attach issue fix---------
+void mjCSkin::ResolveReferences(const mjCModel* m) {
+  // Resize the bodyid list to match the bodyname list
+  bodyid.resize(bodyname_.size());
+
+  for (int i = 0; i < bodyname_.size(); ++i) {
+    if (!bodyname_[i].empty()) {
+      // Find the body object using the (now prefixed) name
+      mjCBody* b = (mjCBody*)m->FindObject(mjOBJ_BODY, bodyname_[i]);
+      if (b) {
+        // Success! Store the numerical ID.
+        bodyid[i] = b->id;
+      } else {
+        // This is the error we are fixing!
+        throw mjCError(this, "unknown body '%s' in skin", bodyname_[i].c_str());
+      }
+    } else {
+      // This bone doesn't reference a body.
+      bodyid[i] = -1;
+    }
+  }
+}
 //------------------ class mjCFrame implementation -------------------------------------------------
 
 // initialize frame