Add Exists and ExistsMulti convenience methods

service/datastore/datastore_test.go

Index: service/datastore/datastore_test.go
diff --git a/service/datastore/datastore_test.go b/service/datastore/datastore_test.go
index 2c2967537051863ab635c181c8884c793e7dac40..d86123f523ef16a5d349cf9ebccd4c9580f9f1a4 100644
--- a/service/datastore/datastore_test.go
+++ b/service/datastore/datastore_test.go
@@ -90,6 +90,8 @@ func (f *fakeDatastore) GetMulti(keys []*Key, _meta MultiMetaGetter, cb GetMulti
 	for i, k := range keys {
 		if k.Kind() == "Fail" {
 			cb(nil, errors.New("GetMulti fail"))
+		} else if k.Kind() == "DNE" {
+			cb(nil, ErrNoSuchEntity)
 		} else {
 			cb(PropertyMap{"Value": {MkProperty(i + 1)}}, nil)
 		}
@@ -298,6 +300,19 @@ func TestKeyForObj(t *testing.T) {
 				So(ds.KeyForObj(pls).String(), ShouldEqual, `s~aid:ns:/Hello,"world"/CommonStruct,1`)
 			})
 
+			Convey("can see if things exist", func() {
+				e, err := ds.Exists(k)
+				So(err, ShouldBeNil)
+				So(e, ShouldBeTrue)
+
+				e, err = ds.Exists(ds.MakeKey("DNE", "nope"))
+				So(err, ShouldBeNil)
+				So(e, ShouldBeFalse)
+
+				_, err = ds.Exists(ds.MakeKey("Fail", "boom"))
+				So(err, ShouldErrLike, "GetMulti fail")
+			})
+
 		})
 
 		Convey("bad", func() {