Don't copy key's datset ID to the protobuf.

The back-end already knows it, and it is actually different than the
'project ID' we are assigning to the dataset as its ID.

Fixes #121.

Author: tseaver

gcloud/datastore/key.py

@@ -91,18 +91,9 @@ def to_protobuf(self):
         """
         key = datastore_pb.Key()
 
-        # Technically a dataset is required to do anything with the key,
-        # but we shouldn't throw a cryptic error if one isn't provided
-        # in the initializer.
-        if self.dataset():
-            # Apparently 's~' is a prefix for High-Replication and is necessary
-            # here. Another valid preflix is 'e~' indicating EU datacenters.
-            dataset_id = self.dataset().id()
-            if dataset_id:
-                if dataset_id[:2] not in ['s~', 'e~']:
-                    dataset_id = 's~' + dataset_id
-
-                key.partition_id.dataset_id = dataset_id
+        # Don't copy the dataset ID to the protobuf:  the backend
+        # already knows the dataset we are working with, and sets
+        # it on returned keys.
 
         if self._namespace:
             key.partition_id.namespace = self._namespace

gcloud/datastore/test_key.py

@@ -139,23 +139,23 @@ def test_to_protobuf_w_explicit_dataset_no_prefix(self):
         dataset = Dataset(_DATASET)
         key = self._makeOne(dataset)
         pb = key.to_protobuf()
-        self.assertEqual(pb.partition_id.dataset_id, 's~%s' % _DATASET)
+        self.assertEqual(pb.partition_id.dataset_id, '')
 
     def test_to_protobuf_w_explicit_dataset_w_s_prefix(self):
         from gcloud.datastore.dataset import Dataset
         _DATASET = 's~DATASET'
         dataset = Dataset(_DATASET)
         key = self._makeOne(dataset)
         pb = key.to_protobuf()
-        self.assertEqual(pb.partition_id.dataset_id, _DATASET)
+        self.assertEqual(pb.partition_id.dataset_id, '')
 
     def test_to_protobuf_w_explicit_dataset_w_e_prefix(self):
         from gcloud.datastore.dataset import Dataset
         _DATASET = 'e~DATASET'
         dataset = Dataset(_DATASET)
         key = self._makeOne(dataset)
         pb = key.to_protobuf()
-        self.assertEqual(pb.partition_id.dataset_id, _DATASET)
+        self.assertEqual(pb.partition_id.dataset_id, '')
 
     def test_to_protobuf_w_explicit_namespace(self):
         _NAMESPACE = 'NAMESPACE'