code: def test_to_json_categorical_index(self):
      data = pd.Series(1, pd.CategoricalIndex(["a", "b"]))
      result = data.to_json(orient="table", date_format="iso")
      result = json.loads(result, object_pairs_hook=OrderedDict)
      result["schema"].pop("pandas_version")
  
      expected = OrderedDict(
          [
              (
                  "schema",
                  {
                      "fields": [
                          {
                              "name": "index",
                              "type": "any",
                              "constraints": {"enum": ["a", "b"]},
                              "ordered": False,
                          },
                          {"name": "values", "type": "integer"},
                      ],
                      "primaryKey": ["index"],
                  },
              ),
              (
                  "data",
                  [
                      OrderedDict([("index", "a"), ("values", 1)]),
                      OrderedDict([("index", "b"), ("values", 1)]),
                  ],
              ),
          ]
      )
  
      assert result == expected
  
imports: from collections import OrderedDict
  from io import StringIO
  import json
  import numpy
  import pytest
  from pandas.core.dtypes.dtypes import CategoricalDtype, DatetimeTZDtype, PeriodDtype
  import pandas
  from pandas import DataFrame
  import pandas._testing
  from pandas.io.json._table_schema import as_json_table_type, build_table_schema, convert_json_field_to_pandas_type, convert_pandas_type_to_json_field, set_default_names
label_desc: Use image node_id 6 for calling the TestTableOrient obj's underlying member method code with example usage: obj.test_to_json_categorical_index() without return types