BUG:pandas-dev#29928 Fix to_json output with 'table' orient for single level MultiIndex.

Index field name in written json was incorrect, so applying read_json resulted in NaN index values. Dataframe to_json with 'table' orient now treats single level MultiIndex like single index.

Author: LucasG0

doc/source/whatsnew/v1.1.0.rst

@@ -793,6 +793,7 @@ I/O
 - Bug in :meth:`~DataFrame.read_feather` was raising an `ArrowIOError` when reading an s3 or http file path (:issue:`29055`)
 - Bug in :meth:`read_parquet` was raising a ``FileNotFoundError`` when passed an s3 directory path. (:issue:`26388`)
 - Bug in :meth:`~DataFrame.to_parquet` was throwing an ``AttributeError`` when writing a partitioned parquet file to s3 (:issue:`27596`)
+- Bug in :meth:`~DataFrame.to_json` with 'table' orient was writting wrong index field name for MultiIndex Dataframe with a single level. (:issue:`29928`)
 
 Plotting
 ^^^^^^^^

pandas/io/json/_json.py

@@ -15,7 +15,7 @@
 
 from pandas.core.dtypes.common import ensure_str, is_period_dtype
 
-from pandas import DataFrame, MultiIndex, Series, isna, to_datetime
+from pandas import DataFrame, Index, MultiIndex, Series, isna, to_datetime
 from pandas.core.construction import create_series_with_explicit_dtype
 from pandas.core.reshape.concat import concat
 
@@ -286,6 +286,9 @@ def __init__(
             )
             raise ValueError(msg)
 
+        if obj.index.nlevels == 1 and isinstance(obj.index, MultiIndex):
+            obj.index = obj.index.get_level_values(0)
+
         self.schema = build_table_schema(obj, index=self.index)
 
         # NotImplemented on a column MultiIndex

pandas/tests/io/json/test_json_table_schema.py

@@ -435,6 +435,21 @@ def test_to_json_categorical_index(self):
 
         assert result == expected
 
+    @pytest.mark.parametrize("name", [None, "foo"])
+    def test_multiindex_single_level(self, name):
+        # GH29928
+        index = pd.Index([1,2,3,4], name=name)
+        df = pd.DataFrame(data=[[1,1],[2,2],[3,3],[4,4]], columns=['A','B'],
+                          index=index)
+        expected = df.to_json(orient='table')
+
+        index = pd.MultiIndex.from_tuples([(1,),(2,),(3,),(4,)], names=[name])
+        df = pd.DataFrame(data=[[1,1],[2,2],[3,3],[4,4]],columns=['A','B'],
+                        index=index)
+        result = df.to_json(orient='table')
+
+        assert result == expected
+
     def test_date_format_raises(self):
         with pytest.raises(ValueError):
             self.df.to_json(orient="table", date_format="epoch")