PYTHON-1665 Agg with $out always goes to primary

doc/changelog.rst

@@ -51,6 +51,9 @@ Version 3.9 adds support for MongoDB 4.2. Highlights include:
   :mod:`~pymongo.monitoring` for an example.
 - :meth:`pymongo.collection.Collection.aggregate` and
   :meth:`pymongo.database.Database.aggregate` now support the ``$merge`` pipeline
+  stage and use read preference
+  :attr:`~pymongo.read_preferences.ReadPreference.PRIMARY` if the ``$out`` or
+  ``$merge`` pipeline stages are used.
 - Support for specifying a pipeline or document in
   :meth:`~pymongo.collection.Collection.update_one`,
   :meth:`~pymongo.collection.Collection.update_many`,

pymongo/aggregation.py

@@ -19,6 +19,7 @@ from bson.son import SON
 from pymongo import common
 from pymongo.collation import validate_collation_or_none
 from pymongo.errors import ConfigurationError
+from pymongo.read_preferences import ReadPreference
 
 
 class _AggregationCommand(object):
@@ -97,6 +98,11 @@ class _AggregationCommand(object):
             self._result_processor(
                 result, session, server, sock_info, slave_ok)
 
+    def get_read_preference(self, session):
+        if self._performs_write:
+            return ReadPreference.PRIMARY
+        return self._target._read_preference_for(session)
+
     def get_cursor(self, session, server, sock_info, slave_ok):
         # Ensure command compatibility.
         self._check_compat(sock_info)
@@ -106,9 +112,6 @@ class _AggregationCommand(object):
                    ("pipeline", self._pipeline)])
         cmd.update(self._options)
 
-        # Cache read preference for easy access.
-        read_preference = self._target._read_preference_for(session)
-
         # Apply this target's read concern if:
         # readConcern has not been specified as a kwarg and either
         # - server version is >= 4.2 or
@@ -134,7 +137,7 @@ class _AggregationCommand(object):
             self._database.name,
             cmd,
             slave_ok,
-            read_preference,
+            self.get_read_preference(session),
             self._target.codec_options,
             parse_write_concern_error=True,
             read_concern=read_concern,

pymongo/collection.py

@@ -2287,7 +2287,7 @@ class Collection(common.BaseObject):
             self, cursor_class, pipeline, kwargs, explicit_session,
             user_fields={'cursor': {'firstBatch': 1}}, use_cursor=use_cursor)
         return self.__database.client._retryable_read(
-            cmd.get_cursor, self._read_preference_for(session), session,
+            cmd.get_cursor, cmd.get_read_preference(session), session,
             retryable=not cmd._performs_write)
 
     def aggregate(self, pipeline, session=None, **kwargs):
@@ -2313,11 +2313,9 @@ class Collection(common.BaseObject):
           - `useCursor` (bool): Deprecated. Will be removed in PyMongo 4.0.
 
         The :meth:`aggregate` method obeys the :attr:`read_preference` of this
-        :class:`Collection`. Please note that using the ``$out`` and ``$merge``
-        pipeline stages requires a read preference of
-        :attr:`~pymongo.read_preferences.ReadPreference.PRIMARY` (the default).
-        The server will raise an error if the ``$out`` or ``$merge`` pipeline
-        stages are used with any other read preference.
+        :class:`Collection`, except when ``$out`` or ``$merge`` are used, in
+        which case  :attr:`~pymongo.read_preferences.ReadPreference.PRIMARY`
+        is used.
 
         .. note:: This method does not support the 'explain' option. Please
            use :meth:`~pymongo.database.Database.command` instead. An
@@ -2337,11 +2335,12 @@ class Collection(common.BaseObject):
           A :class:`~pymongo.command_cursor.CommandCursor` over the result
           set.
 
-        .. versionchanged:: 3.9
-           Added support for the ``$merge`` pipeline stage.
         .. versionchanged:: 3.9
            Apply this collection's read concern to pipelines containing the
            `$out` stage when connected to MongoDB >= 4.2.
+           Added support for the ``$merge`` pipeline stage.
+           Aggregations that write always use read preference
+           :attr:`~pymongo.read_preferences.ReadPreference.PRIMARY`.
         .. versionchanged:: 3.6
            Added the `session` parameter. Added the `maxAwaitTimeMS` option.
            Deprecated the `useCursor` option.

pymongo/database.py

@@ -477,11 +477,9 @@ class Database(common.BaseObject):
             :class:`~pymongo.collation.Collation`.
 
         The :meth:`aggregate` method obeys the :attr:`read_preference` of this
-        :class:`Database`. Please note that using the ``$out`` or ``$merge``
-        pipeline stages requires a read preference of
-        :attr:`~pymongo.read_preferences.ReadPreference.PRIMARY` (the default).
-        The server will raise an error if the ``$out`` or ``$merge`` pipeline
-        stages is used with any other read preference.
+        :class:`Database`, except when ``$out`` or ``$merge`` are used, in
+        which case  :attr:`~pymongo.read_preferences.ReadPreference.PRIMARY`
+        is used.
 
         .. note:: This method does not support the 'explain' option. Please
            use :meth:`~pymongo.database.Database.command` instead.
@@ -512,7 +510,7 @@ class Database(common.BaseObject):
                 self, CommandCursor, pipeline, kwargs, session is not None,
                 user_fields={'cursor': {'firstBatch': 1}})
             return self.client._retryable_read(
-                cmd.get_cursor, self._read_preference_for(s), s,
+                cmd.get_cursor, cmd.get_read_preference(s), s,
                 retryable=not cmd._performs_write)
 
     def watch(self, pipeline=None, full_document='default', resume_after=None,

test/test_read_preferences.py

@@ -468,6 +468,11 @@ class TestCommandAndReadPreference(TestReplicaSetClientBase):
                                'aggregate',
                                [{'$project': {'_id': 1}}])
 
+    def test_aggregate_write(self):
+        self._test_coll_helper(False, self.c.pymongo_test.test,
+                               'aggregate',
+                               [{'$project': {'_id': 1}}, {'$out': "agg_write_test"}])
+
 
 class TestMovingAverage(unittest.TestCase):
     def test_moving_average(self):