Fix v2 structured field selection

changes/3983.bugfix.md

@@ -0,0 +1 @@
+Fixed field selection for Zarr v2 arrays with structured dtypes.

src/zarr/core/array.py

@@ -5397,6 +5397,7 @@ async def _get_selection(
 
     # check fields are sensible
     out_dtype = check_fields(fields, dtype)
+    has_fields = bool(fields)
 
     # setup output buffer
     if out is not None:
@@ -5414,6 +5415,11 @@ async def _get_selection(
             dtype=out_dtype,
             order=order,
         )
+    read_buffer = (
+        prototype.nd_buffer.empty(shape=indexer.shape, dtype=dtype, order=order)
+        if has_fields
+        else out_buffer
+    )
     if product(indexer.shape) > 0:
         # need to use the order from the metadata for v2
         _config = config
@@ -5446,7 +5452,7 @@ async def _get_selection(
                 )
                 for chunk_coords, chunk_selection, out_selection, is_complete_chunk in indexed_chunks
             ],
-            out_buffer,
+            read_buffer,
             drop_axes=indexer.drop_axes,
         )
         if _config.read_missing_chunks is False:
@@ -5464,6 +5470,8 @@ async def _get_selection(
                     f"missing chunks with the fill value.\n"
                     f"Missing chunks:\n{chunks_str}"
                 )
+    if has_fields:
+        out_buffer[...] = read_buffer[fields]
     if isinstance(indexer, BasicIndexer) and indexer.shape == ():
         return out_buffer.as_scalar()
     return out_buffer.as_ndarray_like()

tests/test_v2.py

@@ -1,6 +1,6 @@
 import json
 from pathlib import Path
-from typing import Any, Literal
+from typing import Any, Literal, cast
 
 import numpy as np
 import pytest
@@ -252,6 +252,22 @@ def test_structured_dtype_roundtrip(fill_value: float | bytes, tmp_path: Path) -
     assert (a == za[:]).all()
 
 
+def test_structured_dtype_field_selection(tmp_path: Path) -> None:
+    a = np.array(
+        [("Rex", 9, 81.0), ("Fido", 3, 27.0)],
+        dtype=[("name", "U10"), ("age", "i4"), ("weight", "f4")],
+    )
+    za = zarr.create_array(store=tmp_path / "data.zarr", data=a, zarr_format=2)
+
+    np.testing.assert_array_equal(cast(Any, za)["name"], a["name"])
+    np.testing.assert_array_equal(
+        za.get_basic_selection(slice(1, None), fields="age"), a["age"][1:]
+    )
+    np.testing.assert_array_equal(
+        za.get_basic_selection(Ellipsis, fields=["name", "age"]), a[["name", "age"]]
+    )
+
+
 @pytest.mark.parametrize(
     (
         "fill_value",