Improve Document.meta.shard_key docs (#2099)

This closes #2096. Previous documentation of the shard_key meta attribute was
missing the crucial point that it really only matters if your collection is
sharded over a compound index.

docs/guide/defining-documents.rst

@@ -714,11 +714,16 @@ subsequent calls to :meth:`~mongoengine.queryset.QuerySet.order_by`. ::
 Shard keys
 ==========
 
-If your collection is sharded, then you need to specify the shard key as a tuple,
-using the :attr:`shard_key` attribute of :attr:`~mongoengine.Document.meta`.
-This ensures that the shard key is sent with the query when calling the
-:meth:`~mongoengine.document.Document.save` or
-:meth:`~mongoengine.document.Document.update` method on an existing
+If your collection is sharded by multiple keys, then you can improve shard
+routing (and thus the performance of your application) by specifying the shard
+key, using the :attr:`shard_key` attribute of
+:attr:`~mongoengine.Document.meta`. The shard key should be defined as a tuple.
+
+This ensures that the full shard key is sent with the query when calling
+methods such as :meth:`~mongoengine.document.Document.save`,
+:meth:`~mongoengine.document.Document.update`,
+:meth:`~mongoengine.document.Document.modify`, or
+:meth:`~mongoengine.document.Document.delete` on an existing
 :class:`~mongoengine.Document` instance::
 
     class LogEntry(Document):
@@ -728,7 +733,8 @@ This ensures that the shard key is sent with the query when calling the
         data = StringField()
 
         meta = {
-            'shard_key': ('machine', 'timestamp',)
+            'shard_key': ('machine', 'timestamp'),
+            'indexes': ('machine', 'timestamp'),
         }
 
 .. _document-inheritance:

mongoengine/document.py

@@ -544,7 +544,7 @@ class Document(six.with_metaclass(TopLevelDocumentMetaclass, BaseDocument)):
 
     @property
     def _qs(self):
-        """Return the queryset to use for updating / reloading / deletions."""
+        """Return the default queryset corresponding to this document."""
         if not hasattr(self, '__objects'):
             self.__objects = QuerySet(self, self._get_collection())
         return self.__objects
@@ -552,9 +552,11 @@ class Document(six.with_metaclass(TopLevelDocumentMetaclass, BaseDocument)):
     @property
     def _object_key(self):
         """Get the query dict that can be used to fetch this object from
-        the database. Most of the time it's a simple PK lookup, but in
-        case of a sharded collection with a compound shard key, it can
-        contain a more complex query.
+        the database.
+
+        Most of the time the dict is a simple PK lookup, but in case of
+        a sharded collection with a compound shard key, it can contain a more
+        complex query.
         """
         select_dict = {'pk': self.pk}
         shard_key = self.__class__._meta.get('shard_key', tuple())