Add find_one() method for gridfs.

gridfs/__init__.py

@@ -280,6 +280,36 @@ class GridFS(object):
             name for name in self.__files.distinct("filename")
             if name is not None]
 
+    def find_one(self, spec_or_id=None, *args, **kwargs):
+        """Get a single file from gridfs.
+
+        All arguments to :meth:`find` are also valid arguments for
+        :meth:`find_one`, although any `limit` argument will be
+        ignored. Returns a single gridfs file, or ``None`` if no matching
+        file is found. For example::
+
+            file = fs.find_one({"filename": "lisa.txt"})
+
+        :Parameters:
+
+            - `spec_or_id` (optional): a dictionary specifying
+            the query to be performing OR any other type to be used  as
+            the value for a query for ``"_id"`` in the file collection.
+
+            - `*args` (optional): any additional positional arguments are
+            the same as the arguments to :meth:`find`.
+
+            - `**kwargs` (optional): any additional keyword arguments
+            are the same as the arguments to :meth:`find`.
+        """
+        if spec_or_id is not None and not isinstance(spec_or_id, dict):
+            spec_or_id = {"_id": spec_or_id}
+
+        for file in self.find(spec_or_id, *args, **kwargs):
+            return file
+
+        return None
+
     def find(self, *args, **kwargs):
         """Query GridFS for files.