BQ: Pass selected_fields as a string to tabledata.list. (#4143)

BigQuery was only returning the first column when passing in a list
instead of a comma-separated string.

bigquery/google/cloud/bigquery/client.py

@@ -924,7 +924,8 @@ def list_rows(self, table, selected_fields=None, max_results=None,
 
         params = {}
         if selected_fields is not None:
-            params['selectedFields'] = [f.name for f in selected_fields]
+            params['selectedFields'] = ','.join(
+                [f.name for f in selected_fields])
         if start_index is not None:
             params['startIndex'] = start_index
 

bigquery/tests/system.py

@@ -312,8 +312,9 @@ def test_update_table_schema(self):
             self.assertEqual(found.mode, expected.mode)
 
     @staticmethod
-    def _fetch_single_page(table):
-        iterator = Config.CLIENT.list_rows(table)
+    def _fetch_single_page(table, selected_fields=None):
+        iterator = Config.CLIENT.list_rows(
+            table, selected_fields=selected_fields)
         page = six.next(iterator.pages)
         return list(page)
 
@@ -1236,6 +1237,23 @@ def test_dump_table_w_public_data(self):
         table = Config.CLIENT.get_table(table_ref)
         self._fetch_single_page(table)
 
+    def test_dump_table_w_public_data_selected_fields(self):
+        PUBLIC = 'bigquery-public-data'
+        DATASET_ID = 'samples'
+        TABLE_NAME = 'natality'
+        selected_fields = [
+            bigquery.SchemaField('year', 'INTEGER', mode='NULLABLE'),
+            bigquery.SchemaField('month', 'INTEGER', mode='NULLABLE'),
+            bigquery.SchemaField('day', 'INTEGER', mode='NULLABLE'),
+        ]
+        table_ref = DatasetReference(PUBLIC, DATASET_ID).table(TABLE_NAME)
+
+        rows = self._fetch_single_page(
+            table_ref, selected_fields=selected_fields)
+
+        self.assertGreater(len(rows), 0)
+        self.assertEqual(len(rows[0]), 3)
+
     def test_large_query_w_public_data(self):
         PUBLIC = 'bigquery-public-data'
         DATASET_ID = 'samples'