BUG: fix in categorical merges

doc/source/whatsnew/v1.1.0.rst

@@ -102,7 +102,7 @@ Bug fixes
 Categorical
 ^^^^^^^^^^^
 
--
+- Bug where :func:`merge` was unable to join on non-unique categorical indices (:issue:`28189`)
 -
 
 Datetimelike

pandas/_libs/join.pyx

@@ -242,6 +242,8 @@ ctypedef fused join_t:
     float64_t
     float32_t
     object
+    int8_t
+    int16_t
     int32_t
     int64_t
     uint64_t

pandas/core/indexes/category.py

@@ -28,6 +28,7 @@
 from pandas.core.indexes.base import Index, _index_shared_docs, maybe_extract_name
 from pandas.core.indexes.extension import ExtensionIndex, inherit_names
 import pandas.core.missing as missing
+from pandas.core.ops import get_op_result_name
 
 _index_doc_kwargs = dict(ibase._index_doc_kwargs)
 _index_doc_kwargs.update(dict(target_klass="CategoricalIndex"))
@@ -781,6 +782,12 @@ def _delegate_method(self, name: str, *args, **kwargs):
             return res
         return CategoricalIndex(res, name=self.name)
 
+    def _wrap_joined_index(
+        self, joined: np.ndarray, other: "CategoricalIndex"
+    ) -> "CategoricalIndex":
+        name = get_op_result_name(self, other)
+        return self._create_from_codes(joined, name=name)
+
 
 CategoricalIndex._add_numeric_methods_add_sub_disabled()
 CategoricalIndex._add_numeric_methods_disabled()

pandas/tests/reshape/merge/test_merge.py

@@ -2163,3 +2163,17 @@ def test_merge_datetime_upcast_dtype():
         }
     )
     tm.assert_frame_equal(result, expected)
+
+
+def test_categorical_non_unique_monotonic():
+    # GH 28189
+    df = DataFrame(range(4), columns=["value"], index=CategoricalIndex(["1"] * 4))
+    df2 = DataFrame([[6]], columns=["value"], index=CategoricalIndex(["1"]))
+
+    result = merge(df, df2, how="left", left_index=True, right_index=True)
+    expected = DataFrame(
+        [[0, 6], [1, 6], [2, 6], [3, 6]],
+        columns=["value_x", "value_y"],
+        index=CategoricalIndex(["1"] * 4),
+    )
+    tm.assert_frame_equal(expected, result)