fix: allow partial streaming_buffer statistics (googleapis#37)

* fix: allow partial streaming_buffer statistics.

Previously, the BQ backend would supply all fields of the
streamingBuffer statistics or none.  This is no longer the
case, so we relax construction to not depend on all values
being present.

Related: internal issue b/148720220

google/cloud/bigquery/table.py

@@ -1139,12 +1139,17 @@ class StreamingBuffer(object):
     """
 
     def __init__(self, resource):
-        self.estimated_bytes = int(resource["estimatedBytes"])
-        self.estimated_rows = int(resource["estimatedRows"])
-        # time is in milliseconds since the epoch.
-        self.oldest_entry_time = google.cloud._helpers._datetime_from_microseconds(
-            1000.0 * int(resource["oldestEntryTime"])
-        )
+        self.estimated_bytes = None
+        if "estimatedBytes" in resource:
+            self.estimated_bytes = int(resource["estimatedBytes"])
+        self.estimated_rows = None
+        if "estimatedRows" in resource:
+            self.estimated_rows = int(resource["estimatedRows"])
+        self.oldest_entry_time = None
+        if "oldestEntryTime" in resource:
+            self.oldest_entry_time = google.cloud._helpers._datetime_from_microseconds(
+                1000.0 * int(resource["oldestEntryTime"])
+            )
 
 
 class Row(object):

tests/unit/test_table.py

@@ -855,6 +855,29 @@ def test_from_api_repr_w_properties(self):
         table = klass.from_api_repr(RESOURCE)
         self._verifyResourceProperties(table, RESOURCE)
 
+    def test_from_api_repr_w_partial_streamingbuffer(self):
+        import datetime
+        from google.cloud._helpers import UTC
+        from google.cloud._helpers import _millis
+
+        RESOURCE = self._make_resource()
+        self.OLDEST_TIME = datetime.datetime(2015, 8, 1, 23, 59, 59, tzinfo=UTC)
+        RESOURCE["streamingBuffer"] = {"oldestEntryTime": _millis(self.OLDEST_TIME)}
+        klass = self._get_target_class()
+        table = klass.from_api_repr(RESOURCE)
+        self.assertIsNotNone(table.streaming_buffer)
+        self.assertIsNone(table.streaming_buffer.estimated_rows)
+        self.assertIsNone(table.streaming_buffer.estimated_bytes)
+        self.assertEqual(table.streaming_buffer.oldest_entry_time, self.OLDEST_TIME)
+        # Another partial construction
+        RESOURCE["streamingBuffer"] = {"estimatedRows": 1}
+        klass = self._get_target_class()
+        table = klass.from_api_repr(RESOURCE)
+        self.assertIsNotNone(table.streaming_buffer)
+        self.assertEqual(table.streaming_buffer.estimated_rows, 1)
+        self.assertIsNone(table.streaming_buffer.estimated_bytes)
+        self.assertIsNone(table.streaming_buffer.oldest_entry_time)
+
     def test_from_api_with_encryption(self):
         self._setUpConstants()
         RESOURCE = {