fix: Arrow extension-type metadata was not set when calling the REST API or when there are no rows

google/cloud/bigquery/_pandas_helpers.py

@@ -173,6 +173,13 @@ def pyarrow_timestamp():
         pyarrow.decimal128(38, scale=9).id: "NUMERIC",
         pyarrow.decimal256(76, scale=38).id: "BIGNUMERIC",
     }
+    BQ_FIELD_TYPE_TO_ARROW_FIELD_METADATA = {
+        "GEOGRAPHY": {
+            b"ARROW:extension:name": b"google:sqlType:geography",
+            b"ARROW:extension:metadata": b'{"encoding": "WKT"}',
+        },
+        "DATETIME": {b"ARROW:extension:name": b"google:sqlType:datetime"},
+    }
 
 else:  # pragma: NO COVER
     BQ_TO_ARROW_SCALARS = {}  # pragma: NO COVER
@@ -227,7 +234,12 @@ def bq_to_arrow_field(bq_field, array_type=None):
         if array_type is not None:
             arrow_type = array_type  # For GEOGRAPHY, at least initially
         is_nullable = bq_field.mode.upper() == "NULLABLE"
-        return pyarrow.field(bq_field.name, arrow_type, nullable=is_nullable)
+        metadata = BQ_FIELD_TYPE_TO_ARROW_FIELD_METADATA.get(
+            bq_field.field_type.upper() if bq_field.field_type else ""
+        )
+        return pyarrow.field(
+            bq_field.name, arrow_type, nullable=is_nullable, metadata=metadata
+        )
 
     warnings.warn("Unable to determine type for field '{}'.".format(bq_field.name))
     return None

google/cloud/bigquery/table.py

@@ -1810,7 +1810,7 @@ def to_arrow(
             if owns_bqstorage_client:
                 bqstorage_client._transport.grpc_channel.close()
 
-        if record_batches:
+        if record_batches and bqstorage_client is not None:
             return pyarrow.Table.from_batches(record_batches)
         else:
             # No records, use schema based on BigQuery schema.

tests/system/conftest.py

@@ -13,6 +13,7 @@
 # limitations under the License.
 
 import pathlib
+import re
 
 import pytest
 import test_utils.prefixer
@@ -61,6 +62,17 @@ def dataset_id(bigquery_client):
     bigquery_client.delete_dataset(dataset_id, delete_contents=True, not_found_ok=True)
 
 
+@pytest.fixture()
+def dataset_client(bigquery_client, dataset_id):
+    import google.cloud.bigquery.job
+
+    return bigquery.Client(
+        default_query_job_config=google.cloud.bigquery.job.QueryJobConfig(
+            default_dataset=f"{bigquery_client.project}.{dataset_id}",
+        )
+    )
+
+
 @pytest.fixture
 def table_id(dataset_id):
     return f"{dataset_id}.table_{helpers.temp_suffix()}"
@@ -98,3 +110,8 @@ def scalars_extreme_table(
     job.result()
     yield full_table_id
     bigquery_client.delete_table(full_table_id)
+
+
+@pytest.fixture
+def test_table_name(request, replace_non_anum=re.compile(r"[^a-zA-Z0-9_]").sub):
+    return replace_non_anum("_", request.node.name)

tests/system/test_arrow.py

@@ -110,3 +110,56 @@ def test_list_rows_nullable_scalars_dtypes(
     timestamp_type = schema.field("timestamp_col").type
     assert timestamp_type.unit == "us"
     assert timestamp_type.tz is not None
+
+
+@pytest.mark.parametrize("do_insert", [True, False])
+def test_arrow_extension_types_same_for_storage_and_REST_APIs_894(
+    dataset_client, test_table_name, do_insert
+):
+    types = dict(
+        astring=("STRING", "'x'"),
+        astring9=("STRING(9)", "'x'"),
+        abytes=("BYTES", "b'x'"),
+        abytes9=("BYTES(9)", "b'x'"),
+        anumeric=("NUMERIC", "42"),
+        anumeric9=("NUMERIC(9)", "42"),
+        anumeric92=("NUMERIC(9,2)", "42"),
+        abignumeric=("BIGNUMERIC", "42e30"),
+        abignumeric49=("BIGNUMERIC(37)", "42e30"),
+        abignumeric492=("BIGNUMERIC(37,2)", "42e30"),
+        abool=("BOOL", "true"),
+        adate=("DATE", "'2021-09-06'"),
+        adatetime=("DATETIME", "'2021-09-06T09:57:26'"),
+        ageography=("GEOGRAPHY", "ST_GEOGFROMTEXT('point(0 0)')"),
+        # Can't get arrow data for interval :(
+        # ainterval=('INTERVAL', "make_interval(1, 2, 3, 4, 5, 6)"),
+        aint64=("INT64", "42"),
+        afloat64=("FLOAT64", "42.0"),
+        astruct=("STRUCT<v int64>", "struct(42)"),
+        atime=("TIME", "'1:2:3'"),
+        atimestamp=("TIMESTAMP", "'2021-09-06T09:57:26'"),
+    )
+    columns = ", ".join(f"{k} {t[0]}" for k, t in types.items())
+    dataset_client.query(f"create table {test_table_name} ({columns})").result()
+    if do_insert:
+        names = list(types)
+        values = ", ".join(types[name][1] for name in names)
+        names = ", ".join(names)
+        dataset_client.query(
+            f"insert into {test_table_name} ({names}) values ({values})"
+        ).result()
+    at = dataset_client.query(f"select * from {test_table_name}").result().to_arrow()
+    smd = {at.field(i).name: at.field(i).metadata for i in range(at.num_columns)}
+    at = (
+        dataset_client.query(f"select * from {test_table_name}")
+        .result()
+        .to_arrow(create_bqstorage_client=False)
+    )
+    rmd = {at.field(i).name: at.field(i).metadata for i in range(at.num_columns)}
+
+    assert rmd == smd
+    assert rmd["adatetime"] == {b"ARROW:extension:name": b"google:sqlType:datetime"}
+    assert rmd["ageography"] == {
+        b"ARROW:extension:name": b"google:sqlType:geography",
+        b"ARROW:extension:metadata": b'{"encoding": "WKT"}',
+    }

tests/unit/test__pandas_helpers.py

@@ -1696,3 +1696,26 @@ def test_bq_to_arrow_field_type_override(module_under_test):
         ).type
         == pyarrow.binary()
     )
+
+
+@pytest.mark.skipif(isinstance(pyarrow, mock.Mock), reason="Requires `pyarrow`")
+@pytest.mark.parametrize(
+    "field_type, metadata",
+    [
+        ("datetime", {b"ARROW:extension:name": b"google:sqlType:datetime"}),
+        (
+            "geography",
+            {
+                b"ARROW:extension:name": b"google:sqlType:geography",
+                b"ARROW:extension:metadata": b'{"encoding": "WKT"}',
+            },
+        ),
+    ],
+)
+def test_bq_to_arrow_field_metadata(module_under_test, field_type, metadata):
+    assert (
+        module_under_test.bq_to_arrow_field(
+            schema.SchemaField("g", field_type)
+        ).metadata
+        == metadata
+    )