fix: support root empty field in Parquet file (#1619)

* refactor: remove duplicate loc

* fix: specify column_prefix

* test: add test for #1619

* test: update test to show crash happens with extension arrays

src/awkward/_v2/operations/ak_from_parquet.py

@@ -132,16 +132,18 @@ def metadata(
         ):
             list_indicator = "list.element"
             break
+
+    subform = ak._v2._connect.pyarrow.form_handle_arrow(
+        parquetfile_for_metadata.schema_arrow, pass_empty_field=True
+    )
     if columns is not None:
+        subform = subform.select_columns(columns)
 
-        form = ak._v2._connect.pyarrow.form_handle_arrow(
-            parquetfile_for_metadata.schema_arrow, pass_empty_field=True
-        )
-        subform = form.select_columns(columns)
+    # Handle empty field at root
+    if parquetfile_for_metadata.schema_arrow.names == [""]:
+        column_prefix = ("",)
     else:
-        subform = ak._v2._connect.pyarrow.form_handle_arrow(
-            parquetfile_for_metadata.schema_arrow, pass_empty_field=True
-        )
+        column_prefix = ()
 
     metadata = parquetfile_for_metadata.metadata
     if scan_files and not path_for_schema.endswith("/_metadata"):
@@ -196,7 +198,9 @@ def metadata(
         else:
             col_counts = None
 
-    parquet_columns = subform.columns(list_indicator=list_indicator)
+    parquet_columns = subform.columns(
+        list_indicator=list_indicator, column_prefix=column_prefix
+    )
 
     return parquet_columns, subform, actual_paths, fs, subrg, col_counts, metadata
 

tests/v2/test_1619-from-parquet-empty-field.py

@@ -0,0 +1,31 @@
+# BSD 3-Clause License; see https://github.com/scikit-hep/awkward-1.0/blob/main/LICENSE
+
+import os
+
+import pytest  # noqa: F401
+import numpy as np  # noqa: F401
+import awkward as ak  # noqa: F401
+
+pytest.importorskip("pyarrow.parquet")
+
+
+def test_no_extension(tmp_path):
+    array = ak._v2.Array(
+        [
+            [
+                {"x": 1, "y": 1.1},
+                {"x": 2, "y": 2.2},
+                {"x": 3, "y": 3.3},
+            ],
+            [
+                {"x": 1, "y": 1.1},
+                {"x": 2, "y": 2.2},
+            ],
+        ]
+    )
+    path = os.path.join(tmp_path, "array-no-ext.parquet")
+
+    ak._v2.to_parquet(array, path, extensionarray=False)
+
+    result = ak._v2.from_parquet(path, columns=["x"])
+    assert result.fields == ["x"]