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

@@ -802,6 +802,7 @@ 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

@@ -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 +435,23 @@ 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")