Fix cat.as_ordered not propogating correct size (#15780)

closes #15778

Authors:
  - Matthew Roeschke (https://github.com/mroeschke)
  - GALI PREM SAGAR (https://github.com/galipremsagar)

Approvers:
  - Lawrence Mitchell (https://github.com/wence-)

URL: #15780

python/cudf/cudf/core/column/categorical.py

@@ -1439,7 +1439,7 @@ def as_ordered(self, ordered: bool):
             categories=self.categories,
             codes=self.codes,
             mask=self.base_mask,
-            size=self.base_size,
+            size=self.size,
             offset=self.offset,
             ordered=ordered,
         )

python/cudf/cudf/tests/test_indexing.py

@@ -2349,3 +2349,16 @@ def test_loc_datetime_random_with_ts(data, scalar):
         expected = pdf.loc[:i]
 
         assert_eq(actual, expected)
+
+
+def test_sliced_categorical_as_ordered():
+    df = cudf.DataFrame({"a": list("caba"), "b": list(range(4))})
+    df["a"] = df["a"].astype("category")
+    df = df.iloc[:2]
+    result = df["a"].cat.as_ordered()
+    expected = cudf.Series(
+        ["c", "a"],
+        dtype=cudf.CategoricalDtype(list("abc"), ordered=True),
+        name="a",
+    )
+    assert_eq(result, expected)