test: look for transaction info on child job, not parent job (#978)

* test: look for transaction info on child job, not parent job

* clarify transaction_info docstring

* use sphinx method directive

Author: tswast

google/cloud/bigquery/job/base.py

@@ -357,6 +357,11 @@ def reservation_usage(self):
     def transaction_info(self) -> Optional[TransactionInfo]:
         """Information of the multi-statement transaction if this job is part of one.
 
+        Since a scripting query job can execute multiple transactions, this
+        property is only expected on child jobs. Use the
+        :meth:`google.cloud.bigquery.client.Client.list_jobs` method with the
+        ``parent_job`` parameter to iterate over child jobs.
+
         .. versionadded:: 2.24.0
         """
         info = self._properties.get("statistics", {}).get("transactionInfo")

tests/system/test_client.py

@@ -1586,9 +1586,15 @@ def test_transaction_info(self):
         query_job = Config.CLIENT.query(sql)
         query_job.result()
 
-        # Transaction ID set by the server should be accessible
-        assert query_job.transaction_info is not None
-        assert query_job.transaction_info.transaction_id != ""
+        child_jobs = Config.CLIENT.list_jobs(parent_job=query_job)
+        begin_transaction_job = next(iter(child_jobs))
+
+        # Transaction ID set by the server should be accessible on the child
+        # job responsible for `BEGIN TRANSACTION`. It is not expected to be
+        # present on the parent job itself.
+        # https://github.com/googleapis/python-bigquery/issues/975
+        assert begin_transaction_job.transaction_info is not None
+        assert begin_transaction_job.transaction_info.transaction_id != ""
 
     def test_dbapi_w_standard_sql_types(self):
         for sql, expected in helpers.STANDARD_SQL_EXAMPLES: