Serialization of nested models fails when nested model pose has "relative_to"

[azeey]

Environment

• OS Version:
• Source or binary build? source, main, fa09864f759f4b267b60ca4ea281fb6ce69d2e1a

Description

• Expected behavior: No errors when running a world with nested models
• Actual behavior: Serialization errors are printed when running a world with nested models where the pose of a model is given relative to another entity.

Steps to reproduce

1. Run the following world with ign gazebo -v1

   <?xml version="1.0" ?>
   <sdf version="1.8">
   <world name="nested_model_world">
   <model name="model_00">
     <link name="link_00"/>
   
     <model name="model_01">
       <pose relative_to="link_00">1 0 0.0 0 0 0</pose> <!-- Causes serialization errors -->
       <link name="link_01"/>
     </model>
   </model>
   </world>
   </sdf>

Output

Error:   Could not find the 'robot' element in the xml file
         at line 80 in /build/urdfdom-YMMa9X/urdfdom-1.0.0/urdf_parser/src/model.cpp
Error [parser_urdf.cc:3255] Unable to call parseURDF on robot model
Error [parser.cc:820] parse as old deprecated model file failed.
[GUI] [Err] [Model.hh:73] Unable to unserialize sdf::Model

I printed out the received SDFormat string for deserialization in serializers::SdfModelSerializer::Deserialize and ran it under ign sdf -k:

Error:   Could not find the 'robot' element in the xml file
         at line 80 in /build/urdfdom-YMMa9X/urdfdom-1.0.0/urdf_parser/src/model.cpp
Error Code 5: [:/home/addisu/ws/fortress/src/ign-gazebo/test.sdf:L2]: Msg: Attribute //pose[@relative_to] of top level model must be left empty, found //pose[@relative_to='link_00'].

Error Code 9: Msg: Errors were found when checking the XML of element<sdf>.
Error Code 1: Msg: Unable to read file:/home/addisu/ws/fortress/src/ign-gazebo/test.sdf

[asreejith-vcrs]

Is this something that's still being worked on? Is there a workaround for this?