Allowed to switch databases for a specific query.

mongoengine/context_managers.py

@@ -1,6 +1,5 @@
 from mongoengine.common import _import_class
 from mongoengine.connection import DEFAULT_CONNECTION_NAME, get_db
-from mongoengine.queryset import QuerySet
 
 
 __all__ = ("switch_db", "switch_collection", "no_dereference",
@@ -162,12 +161,6 @@ class no_sub_classes(object):
         return self.cls
 
 
-class QuerySetNoDeRef(QuerySet):
-    """Special no_dereference QuerySet"""
-    def __dereference(items, max_depth=1, instance=None, name=None):
-            return items
-
-
 class query_counter(object):
     """ Query_counter context manager to get the number of queries. """
 

mongoengine/queryset/base.py

@@ -13,11 +13,11 @@ import pymongo
 from pymongo.common import validate_read_preference
 
 from mongoengine import signals
+from mongoengine.context_managers import switch_db
 from mongoengine.common import _import_class
 from mongoengine.base.common import get_document
 from mongoengine.errors import (OperationError, NotUniqueError,
                                 InvalidQueryError, LookUpError)
-
 from mongoengine.queryset import transform
 from mongoengine.queryset.field_list import QueryFieldList
 from mongoengine.queryset.visitor import Q, QNode
@@ -522,6 +522,19 @@ class BaseQuerySet(object):
 
         return self
 
+    def using(self, alias):
+        """This method is for controlling which database the QuerySet will be evaluated against if you are using more than one database.
+
+        :param alias: The database alias
+
+        .. versionadded:: 0.8
+        """
+
+        with switch_db(self._document, alias) as cls:
+            collection = cls._get_collection()
+
+        return self.clone_into(self.__class__(self._document, collection))
+
     def clone(self):
         """Creates a copy of the current
           :class:`~mongoengine.queryset.QuerySet`
@@ -1450,6 +1463,7 @@ class BaseQuerySet(object):
                     # type of this field and use the corresponding
                     # .to_python(...)
                     from mongoengine.fields import EmbeddedDocumentField
+
                     obj = self._document
                     for chunk in path.split('.'):
                         obj = getattr(obj, chunk, None)
@@ -1460,6 +1474,7 @@ class BaseQuerySet(object):
                     if obj and data is not None:
                         data = obj.to_python(data)
             return data
+
         return clean(row)
 
     def _sub_js_fields(self, code):
@@ -1468,6 +1483,7 @@ class BaseQuerySet(object):
         substituted for the MongoDB name of the field (specified using the
         :attr:`name` keyword argument in a field's constructor).
         """
+
         def field_sub(match):
             # Extract just the field name, and look up the field objects
             field_name = match.group(1).split('.')

mongoengine/queryset/queryset.py

@@ -155,3 +155,10 @@ class QuerySetNoCache(BaseQuerySet):
             queryset = self.clone()
         queryset.rewind()
         return queryset
+
+
+class QuerySetNoDeRef(QuerySet):
+    """Special no_dereference QuerySet"""
+
+    def __dereference(items, max_depth=1, instance=None, name=None):
+        return items

tests/queryset/queryset.py

@@ -29,6 +29,7 @@ class QuerySetTest(unittest.TestCase):
 
     def setUp(self):
         connect(db='mongoenginetest')
+        connect(db='mongoenginetest2', alias='test2')
 
         class PersonMeta(EmbeddedDocument):
             weight = IntField()
@@ -2957,6 +2958,21 @@ class QuerySetTest(unittest.TestCase):
 
         Number.drop_collection()
 
+    def test_using(self):
+        """Ensure that switching databases for a queryset is possible
+        """
+        class Number2(Document):
+            n = IntField()
+
+        Number2.drop_collection()
+
+        for i in xrange(1, 10):
+            t = Number2(n=i)
+            t.switch_db('test2')
+            t.save()
+
+        self.assertEqual(len(Number2.objects.using('test2')), 9)
+
     def test_unset_reference(self):
         class Comment(Document):
             text = StringField()