Include topology vars when subsetting with .cf

cf_xarray/accessor.py

@@ -2269,6 +2269,8 @@ def get_associated_variable_names(
 
         if grid := attrs_or_encoding.get("grid", None):
             coords["grid"] = [grid]
+            if isinstance(self._obj, Dataset):
+                coords["coordinates"].extend(sgrid.get_topology_coords(self._obj, grid))
 
         if grid_mapping_attr := attrs_or_encoding.get("grid_mapping", None):
             # Parse grid mapping variables and their coordinates

cf_xarray/sgrid.py

@@ -11,6 +11,31 @@
     # "edge3_dimensions",
 ]
 
+SGRID_COORD_ATTRS = [
+    "node_coordinates",
+    "face_coordinates",
+    "edge1_coordinates",
+    "edge2_coordinates",
+    "volume_coordinates",
+]
+
+
+def get_topology_coords(ds, grid_var_name):
+    """Return coordinate variable names referenced by an SGRID topology variable.
+
+    Reads ``node_coordinates``, ``face_coordinates``, ``edge{1,2}_coordinates``,
+    and ``volume_coordinates`` from the topology variable's attrs and filters
+    to names that are actually present in ``ds``.
+    """
+    if grid_var_name not in ds.variables:
+        return []
+    grid_attrs = ds[grid_var_name].attrs
+    names: list[str] = []
+    for attr_name in SGRID_COORD_ATTRS:
+        if coord_str := grid_attrs.get(attr_name):
+            names.extend(n for n in coord_str.split() if n in ds.variables)
+    return names
+
 
 def parse_axes(ds):
     import re

cf_xarray/tests/test_accessor.py

@@ -2484,6 +2484,43 @@ def test_sgrid():
     }
 
 
+def test_sgrid_includes_topology_coordinates():
+    """Variables referenced in node/face/edge/volume_coordinates of the
+    grid_topology variable should be pulled in by ds.cf[[var]]."""
+    roms = sgrid_roms.copy()
+    for pos in ("psi", "rho", "u", "v"):
+        roms[f"lon_{pos}"] = ((f"xi_{pos}", f"eta_{pos}"), np.zeros((2, 2)))
+        roms[f"lat_{pos}"] = ((f"xi_{pos}", f"eta_{pos}"), np.zeros((2, 2)))
+
+    expected_coord_vars = {
+        "lon_psi",
+        "lat_psi",
+        "lon_rho",
+        "lat_rho",
+        "lon_u",
+        "lat_u",
+        "lon_v",
+        "lat_v",
+    }
+    assoc = roms.cf.get_associated_variable_names("u")
+    assert expected_coord_vars.issubset(set(assoc["coordinates"]))
+
+    subset = roms.cf[["u"]]
+    assert "grid" in subset.variables
+    assert expected_coord_vars.issubset(set(subset.variables))
+
+    # only dim-compatible coords attach to the DataArray form
+    u_da = roms.cf["u"]
+    assert {"lon_u", "lat_u"}.issubset(set(u_da.coords))
+
+    delft = sgrid_delft.copy()
+    delft["node_lon"] = (("inode", "jnode"), np.zeros((2, 2)))
+    delft["node_lat"] = (("inode", "jnode"), np.zeros((2, 2)))
+    delft["foo"] = (("icell", "jcell"), np.ones((2, 2)), {"grid": "grid"})
+    delft_subset = delft.cf[["foo"]]
+    assert {"grid", "node_lon", "node_lat"}.issubset(set(delft_subset.variables))
+
+
 def test_ancillary_variables_extra_dim():
     ds = xr.Dataset(
         {

doc/sgrid_ugrid.md

@@ -61,6 +61,23 @@ only `xi_u`, `eta_u` are listed in the repr even though the attributes on the `g
 variable `grid` list many more dimension names.
 ```
 
+### Coordinate variables
+
+`cf_xarray` also follows the `node_coordinates`, `face_coordinates`,
+`edge1_coordinates`, `edge2_coordinates`, and `volume_coordinates` attributes
+on the `grid_topology` variable. When you select a data variable that
+references a `grid_topology` via its `grid` attribute, the referenced
+coordinate variables are pulled in alongside it:
+
+```python
+ds.cf[["u"]]  # includes `grid`, lon_psi/lat_psi, lon_rho/lat_rho, ...
+```
+
+Only names actually present in the dataset are propagated. For the
+`DataArray` form (`ds.cf["u"]`) xarray only attaches coordinates whose
+dimensions are compatible with the variable, so e.g. only `lon_u`/`lat_u`
+appear as coords on `u`.
+
 ## UGRID
 
 ### Topology variable