Added Custom Objects Managers

Managers can now be directly declared in a Document eg::

```python
    class CustomQuerySetManager(QuerySetManager):

        @staticmethod
        def get_queryset(doc_cls, queryset):
            return queryset(is_published=True)

    class Post(Document):
        is_published = BooleanField(default=False)
        published = CustomQuerySetManager()
```

Refactored the name of the `_manager_func` to `get_queryset` to mark it as
part the public API.  If declaring a Manager with a get_queryset method, it
should be a staticmethod, that accepts the document_class and the queryset.

Note - you can still use decorators in fact in the example below,
we effectively do the same thing as the first example and is much less verbose.

```python

    class Post(Document):
        is_published = BooleanField(default=False)

        @queryset_manager
        def published(doc_cls, queryset):
            return queryset(is_published=True)
```

Thanks to @theojulienne for the initial impetus and code sample #108

mongoengine/base.py

@@ -299,8 +299,10 @@ class TopLevelDocumentMetaclass(DocumentMetaclass):
         new_class = super_new(cls, name, bases, attrs)
 
         # Provide a default queryset unless one has been manually provided
-        if not hasattr(new_class, 'objects'):
-            new_class.objects = QuerySetManager()
+        manager = attrs.get('objects', QuerySetManager())
+        if hasattr(manager, 'queryset_class'):
+            meta['queryset_class'] = manager.queryset_class
+        new_class.objects = manager
 
         user_indexes = [QuerySet._build_index_spec(new_class, spec)
                         for spec in meta['indexes']] + base_indexes

mongoengine/queryset.py

@@ -428,8 +428,6 @@ class QuerySet(object):
             querying collection
         :param query: Django-style query keyword arguments
         """
-        #if q_obj:
-            #self._where_clause = q_obj.as_js(self._document)
         query = Q(**query)
         if q_obj:
             query &= q_obj
@@ -1308,8 +1306,11 @@ class QuerySet(object):
 
 class QuerySetManager(object):
 
-    def __init__(self, manager_func=None):
-        self._manager_func = manager_func
+    get_queryset = None
+
+    def __init__(self, queryset_func=None):
+        if queryset_func:
+            self.get_queryset = queryset_func
         self._collections = {}
 
     def __get__(self, instance, owner):
@@ -1353,11 +1354,11 @@ class QuerySetManager(object):
         # owner is the document that contains the QuerySetManager
         queryset_class = owner._meta['queryset_class'] or QuerySet
         queryset = queryset_class(owner, self._collections[(db, collection)])
-        if self._manager_func:
-            if self._manager_func.func_code.co_argcount == 1:
-                queryset = self._manager_func(queryset)
+        if self.get_queryset:
+            if self.get_queryset.func_code.co_argcount == 1:
+                queryset = self.get_queryset(queryset)
             else:
-                queryset = self._manager_func(owner, queryset)
+                queryset = self.get_queryset(owner, queryset)
         return queryset
 
 

tests/queryset.py

@@ -5,8 +5,9 @@ import unittest
 import pymongo
 from datetime import datetime, timedelta
 
-from mongoengine.queryset import (QuerySet, MultipleObjectsReturned,
-                                  DoesNotExist, QueryFieldList)
+from mongoengine.queryset import (QuerySet, QuerySetManager,
+                                  MultipleObjectsReturned, DoesNotExist,
+                                  QueryFieldList)
 from mongoengine import *
 
 
@@ -1737,6 +1738,53 @@ class QuerySetTest(unittest.TestCase):
 
         Post.drop_collection()
 
+    def test_custom_querysets_set_manager_directly(self):
+        """Ensure that custom QuerySet classes may be used.
+        """
+
+        class CustomQuerySet(QuerySet):
+            def not_empty(self):
+                return len(self) > 0
+
+        class CustomQuerySetManager(QuerySetManager):
+            queryset_class = CustomQuerySet
+
+        class Post(Document):
+            objects = CustomQuerySetManager()
+
+        Post.drop_collection()
+
+        self.assertTrue(isinstance(Post.objects, CustomQuerySet))
+        self.assertFalse(Post.objects.not_empty())
+
+        Post().save()
+        self.assertTrue(Post.objects.not_empty())
+
+        Post.drop_collection()
+
+    def test_custom_querysets_managers_directly(self):
+        """Ensure that custom QuerySet classes may be used.
+        """
+
+        class CustomQuerySetManager(QuerySetManager):
+
+            @staticmethod
+            def get_queryset(doc_cls, queryset):
+                return queryset(is_published=True)
+
+        class Post(Document):
+            is_published = BooleanField(default=False)
+            published = CustomQuerySetManager()
+
+        Post.drop_collection()
+
+        Post().save()
+        Post(is_published=True).save()
+        self.assertEquals(Post.objects.count(), 2)
+        self.assertEquals(Post.published.count(), 1)
+
+        Post.drop_collection()
+
     def test_call_after_limits_set(self):
         """Ensure that re-filtering after slicing works
         """