Allow setting read_concern

mongoengine/context_managers.py

@@ -257,3 +257,11 @@ def set_write_concern(collection, write_concerns):
     combined_concerns = dict(collection.write_concern.document.items())
     combined_concerns.update(write_concerns)
     yield collection.with_options(write_concern=WriteConcern(**combined_concerns))
+
+
+@contextmanager
+def set_read_write_concern(collection, write_concerns, read_concern):
+    combined_write_concerns = dict(collection.write_concern.document.items())
+    combined_write_concerns.update(write_concerns)
+
+    yield collection.with_options(write_concern=WriteConcern(**combined_write_concerns), read_concern=read_concern)

mongoengine/queryset/base.py

@@ -11,6 +11,7 @@ import pymongo
 import pymongo.errors
 from pymongo.collection import ReturnDocument
 from pymongo.common import validate_read_preference
+from pymongo.read_concern import ReadConcern
 import six
 from six import iteritems
 
@@ -18,7 +19,7 @@ from mongoengine import signals
 from mongoengine.base import get_document
 from mongoengine.common import _import_class
 from mongoengine.connection import get_db
-from mongoengine.context_managers import set_write_concern, switch_db
+from mongoengine.context_managers import set_write_concern, set_read_write_concern, switch_db
 from mongoengine.errors import (
     BulkWriteError,
     InvalidQueryError,
@@ -62,6 +63,7 @@ class BaseQuerySet(object):
         self._timeout = True
         self._slave_okay = False
         self._read_preference = None
+        self._read_concern = None
         self._iter = False
         self._scalar = []
         self._none = False
@@ -490,7 +492,7 @@ class BaseQuerySet(object):
                 return result.deleted_count
 
     def update(
-        self, upsert=False, multi=True, write_concern=None, full_result=False, **update
+        self, upsert=False, multi=True, write_concern=None, read_concern=None, full_result=False, **update
     ):
         """Perform an atomic update on the fields matched by the query.
 
@@ -502,6 +504,7 @@ class BaseQuerySet(object):
             ``save(..., write_concern={w: 2, fsync: True}, ...)`` will
             wait until at least two servers have recorded the write and
             will force an fsync on the primary server.
+        :param read_concern: Override the read concern for the operation
         :param full_result: Return the associated ``pymongo.UpdateResult`` rather than just the number
             updated items
         :param update: Django-style update keyword arguments
@@ -528,7 +531,7 @@ class BaseQuerySet(object):
             else:
                 update["$set"] = {"_cls": queryset._document._class_name}
         try:
-            with set_write_concern(queryset._collection, write_concern) as collection:
+            with set_read_write_concern(queryset._collection, write_concern, read_concern) as collection:
                 update_func = collection.update_one
                 if multi:
                     update_func = collection.update_many
@@ -545,7 +548,7 @@ class BaseQuerySet(object):
                 raise OperationError(message)
             raise OperationError(u"Update failed (%s)" % six.text_type(err))
 
-    def upsert_one(self, write_concern=None, **update):
+    def upsert_one(self, write_concern=None, read_concern=None, **update):
         """Overwrite or add the first document matched by the query.
 
         :param write_concern: Extra keyword arguments are passed down which
@@ -554,6 +557,7 @@ class BaseQuerySet(object):
             ``save(..., write_concern={w: 2, fsync: True}, ...)`` will
             wait until at least two servers have recorded the write and
             will force an fsync on the primary server.
+        :param read_concern: Override the read concern for the operation
         :param update: Django-style update keyword arguments
 
         :returns the new or overwritten document
@@ -565,6 +569,7 @@ class BaseQuerySet(object):
             multi=False,
             upsert=True,
             write_concern=write_concern,
+            read_concern=read_concern,
             full_result=True,
             **update
         )
@@ -1196,6 +1201,20 @@ class BaseQuerySet(object):
         queryset._cursor_obj = None  # we need to re-create the cursor object whenever we apply read_preference
         return queryset
 
+    def read_concern(self, read_concern):
+        """Change the read_concern when querying.
+
+        :param read_concern: override ReplicaSetConnection-level
+            preference.
+        """
+        if read_concern is not None and not isinstance(read_concern, ReadConcern):
+            raise TypeError("%r is not a read concern." % (read_concern,))
+
+        queryset = self.clone()
+        queryset._read_concern = read_concern
+        queryset._cursor_obj = None  # we need to re-create the cursor object whenever we apply read_concern
+        return queryset
+    
     def scalar(self, *fields):
         """Instead of returning Document instances, return either a specific
         value or a tuple of values in order.
@@ -1642,9 +1661,9 @@ class BaseQuerySet(object):
         # XXX In PyMongo 3+, we define the read preference on a collection
         # level, not a cursor level. Thus, we need to get a cloned collection
         # object using `with_options` first.
-        if self._read_preference is not None:
+        if self._read_preference is not None or self._read_concern is not None:
             self._cursor_obj = self._collection.with_options(
-                read_preference=self._read_preference
+                read_preference=self._read_preference, read_concern=self._read_concern
             ).find(self._query, **self._cursor_args)
         else:
             self._cursor_obj = self._collection.find(self._query, **self._cursor_args)