TEST: join df with categorical multiIndex (pandas-dev#51088)

* TEST: join df with categorical multiIndex

* compare result and expected DataFrame

* add pytest.mark.parameterize to the test

* use dataframes from the original example

Author: natmokval

pandas/tests/reshape/merge/test_join.py

@@ -956,3 +956,43 @@ def test_join_empty(left_empty, how, exp):
             expected = expected.rename_axis("A")
 
     tm.assert_frame_equal(result, expected)
+
+
+@pytest.mark.parametrize(
+    "how, values",
+    [
+        ("inner", [0, 1, 2]),
+        ("outer", [0, 1, 2]),
+        ("left", [0, 1, 2]),
+        ("right", [0, 2, 1]),
+    ],
+)
+def test_join_multiindex_categorical_output_index_dtype(how, values):
+    # GH#50906
+    df1 = DataFrame(
+        {
+            "a": Categorical([0, 1, 2]),
+            "b": Categorical([0, 1, 2]),
+            "c": [0, 1, 2],
+        }
+    ).set_index(["a", "b"])
+
+    df2 = DataFrame(
+        {
+            "a": Categorical([0, 2, 1]),
+            "b": Categorical([0, 2, 1]),
+            "d": [0, 2, 1],
+        }
+    ).set_index(["a", "b"])
+
+    expected = DataFrame(
+        {
+            "a": Categorical(values),
+            "b": Categorical(values),
+            "c": values,
+            "d": values,
+        }
+    ).set_index(["a", "b"])
+
+    result = df1.join(df2, how=how)
+    tm.assert_frame_equal(result, expected)