TST: Test for MultiIndex merge with CategoricalIndex (pandas-dev#36973)

Author: tim-tran

pandas/tests/reshape/merge/test_merge.py

@@ -2542,3 +2542,32 @@ def test_mergeerror_on_left_index_mismatched_dtypes():
     df_2 = DataFrame(data=["X"], columns=["C"], index=[999])
     with pytest.raises(MergeError, match="Can only pass argument"):
         merge(df_1, df_2, on=["C"], left_index=True)
+
+
+def test_multiindex_merge_with_unordered_categoricalindex():
+    # GH 36973
+    pcat = CategoricalDtype(categories=["P2", "P1"], ordered=False)
+    df1 = DataFrame(
+        {
+            "id": ["C", "C", "D"],
+            "p": Categorical(["P2", "P1", "P2"], dtype=pcat),
+            "a": [0, 1, 2],
+        }
+    ).set_index(["id", "p"])
+    df2 = DataFrame(
+        {
+            "id": ["A", "C", "C"],
+            "p": Categorical(["P2", "P2", "P1"], dtype=pcat),
+            "d1": [10, 11, 12],
+        }
+    ).set_index(["id", "p"])
+    result = merge(df1, df2, how="left", left_index=True, right_index=True)
+    expected = DataFrame(
+        {
+            "id": ["C", "C", "D"],
+            "p": Categorical(["P2", "P1", "P2"], dtype=pcat),
+            "a": [0, 1, 2],
+            "d1": [11.0, 12.0, np.nan],
+        }
+    ).set_index(["id", "p"])
+    tm.assert_frame_equal(result, expected)