BUG: Add horizontal_datum_name for geographic CRS in CRS.to_cf

Author: snowman2

docs/history.rst

@@ -13,6 +13,7 @@ Latest
 - REF: Raise error when :meth:`.CRS.to_wkt`, :meth:`.CRS.to_json`, or :meth:`.CRS.to_proj4` returns None (issue #1036)
 - CLN: Remove `AzumuthalEquidistantConversion` & :class:`LambertAzumuthalEqualAreaConversion`. :class:`AzimuthalEquidistantConversion` & :class:`LambertAzimuthalEqualAreaConversion` should be used instead (pull #1219)
 - BUG: Fix Derived Projected CRS support (issue #1182)
+- BUG: Add horizontal_datum_name for geographic CRS in :meth:`.CRS.to_cf`` (issue #1251)
 
 3.4.1
 -----

pyproj/crs/crs.py

@@ -693,6 +693,8 @@ def to_cf(
         # handle geographic CRS
         if self.geodetic_crs:
             cf_dict["geographic_crs_name"] = self.geodetic_crs.name
+            if self.geodetic_crs.datum:
+                cf_dict["horizontal_datum_name"] = self.geodetic_crs.datum.name
 
         if self.is_geographic:
             if self.coordinate_operation:
@@ -711,8 +713,6 @@ def to_cf(
                         self.coordinate_operation.method_name.lower()
                     ](self.coordinate_operation)
                 )
-                if self.datum:
-                    cf_dict["horizontal_datum_name"] = self.datum.name
             else:
                 cf_dict["grid_mapping_name"] = "latitude_longitude"
             return cf_dict

test/crs/test_crs_cf.py

@@ -221,6 +221,7 @@ def test_cf_from_latlon():
         "grid_mapping_name": "latitude_longitude",
         "geographic_crs_name": "undefined",
         "reference_ellipsoid_name": "undefined",
+        "horizontal_datum_name": "undefined",
     }
     cf_dict = crs.to_cf()
     assert cf_dict.pop("crs_wkt").startswith("GEOGCRS[")
@@ -253,6 +254,7 @@ def test_cf_from_latlon__named():
         "reference_ellipsoid_name": "WGS 84",
         "longitude_of_prime_meridian": 0.0,
         "prime_meridian_name": "Greenwich",
+        "horizontal_datum_name": "World Geodetic System 1984 ensemble",
         "geographic_crs_name": "WGS 84",
         "grid_mapping_name": "latitude_longitude",
     }