Fix for including mesh size fields in .geo_unrolled file and SetFac…

…tory() (#858)

* Fix for mesh size fields in `.geo_unrolled` file and SetFactory()

* Bug fix with geo_unrolled when fragmenting

qiskit_metal/renderers/renderer_gmsh/gmsh_renderer.py

@@ -808,8 +808,9 @@ def fragment_interfaces(self, draw_sample_holder: bool):
             fragmented_geoms = gmsh.model.occ.fragment([object_dimtag],
                                                        all_geom_dimtags)
 
-        all_geom_dimtags.insert(0, object_dimtag)
         updated_geoms = fragmented_geoms[0]
+        insert_idx = updated_geoms.index(object_dimtag)
+        all_geom_dimtags.insert(insert_idx, object_dimtag)
         all_dicts = {
             0: self.paths_dict,
             1: self.polys_dict,
@@ -1233,8 +1234,24 @@ def export_geo_unrolled(self, filepath: str):
         if not os.path.exists(par_dir):
             raise ValueError(f"Directory not found: {par_dir}")
 
+        has_mesh = False if len(gmsh.model.mesh.field.list()) == 0 else True
+        if has_mesh:
+            self.logger.warning(
+                "WARNING: The existing model contains mesh size field definitions, "
+                "which will show up in your exported .geo_unrolled file. If "
+                "you aren't explicitly handling the mesh size fields, we recommend "
+                "to export the geometry before generating the mesh in your design as "
+                "it might interfere with your .geo_unrolled file imports.")
+
         gmsh.write(filepath)
 
+        # Prepend "SetFactory("OpenCASCADE");" in the exported file
+        line = 'SetFactory("OpenCASCADE");'
+        with open(filepath, 'r+') as f:
+            content = f.read()
+            f.seek(0, 0)
+            f.write(line.rstrip('\r\n') + '\n' + content)
+
     def import_post_processing_data(self,
                                     filename: str,
                                     launch_gui: bool = True,