Include UGRID topology vars when subsetting with .cf#648
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Follow the UGRID `mesh` attribute when subsetting with `ds.cf[[var]]`, the UGRID analogue of #640. The referenced `mesh_topology` variable, its connectivity variables (`face_node_connectivity`, `edge_node_connectivity`, ...), and its coordinate variables (`node_coordinates`, `edge_coordinates`, `face_coordinates`) are now pulled in alongside the data variable. Only names present in the dataset are propagated.
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Follows the UGRID
meshattribute when subsetting withds.cf[[var]]— the UGRID analogue of #640 (which did this for SGRIDgrid_topology).When a data variable references a
mesh_topologyvariable via itsmeshattribute,ds.cf[[var]]now pulls in:mesh_topologyvariable itself,face_node_connectivity,edge_node_connectivity,face_edge_connectivity,face_face_connectivity,edge_face_connectivity,boundary_node_connectivity), andnode_coordinates,edge_coordinates,face_coordinates).Only names actually present in the dataset are propagated. The DataArray form (
ds.cf[var]) attaches only dim-compatible coordinates, as with the SGRID/grid_mapping paths.Implementation
cf_xarray/ugrid.pywithget_mesh_variables(ds, mesh_var_name)returning(connectivity, coordinates), mirroringsgrid.get_topology_coords.get_associated_variable_namesgains ameshkey and follows themeshattribute.Motivation
Downstream grid-detection code (e.g. xpublish-tiles) currently works around this by detecting on the full dataset whenever a variable points at a UGRID mesh, because
ds.cf[[var]]drops the mesh topology, connectivity, and node coordinates. This makes the cf subset self-contained for UGRID, matching SGRID behavior.