Check compatibility of coordinate variable dimensions before assignment. (#484)

Closes #428

Author: dcherian

cf_xarray/accessor.py

@@ -859,11 +859,14 @@ def check_results(names, key):
 
         if scalar_key:
             if len(allnames) == 1:
-                da: DataArray = ds.reset_coords()[allnames[0]]  # type: ignore
-                if allnames[0] in coords:
-                    coords.remove(allnames[0])
+                (name,) = allnames
+                da: DataArray = ds.reset_coords()[name]  # type: ignore
+                if name in coords:
+                    coords.remove(name)
                 for k1 in coords:
-                    da.coords[k1] = ds.variables[k1]
+                    var = ds.variables[k1]
+                    if set(var.dims) <= set(da.dims):
+                        da.coords[k1] = ds.variables[k1]
                 return da
             else:
                 raise KeyError(

cf_xarray/tests/test_accessor.py

@@ -2036,3 +2036,16 @@ def test_sgrid():
         "Y": {"jface", "jnode"},
         "Z": {"kface", "knode"},
     }
+
+
+def test_ancillary_variables_extra_dim():
+    ds = xr.Dataset(
+        {
+            "x": (
+                "x",
+                range(10),
+                {"axis": "X", "ancillary_variables": "position_flag"},
+            ),
+        }
+    )
+    assert_identical(ds.cf["X"], ds["x"])