mstate: state package replacement using mgo.

mstate/machine.go

 package mstate
 
 import (
         "fmt"
         "launchpad.net/mgo/bson"
         "strconv"
 )
 
 // Machine represents the state of a machine.
 type Machine struct {
@@ skipping 17 matching lines @@
         mdoc := &machineDoc{}
         err := m.st.machines.Find(bson.D{{"_id", m.id}}).One(mdoc)
         if err != nil {
                 return "", fmt.Errorf("can't get instance id of machine %s: %v", m, err)
         }
         return mdoc.InstanceId, nil
 }
 
 // SetInstanceId sets the provider specific machine id for this machine.
 func (m *Machine) SetInstanceId(id string) error {
-        err := m.st.machines.Update(bson.D{{"_id", m.id}}, bson.D{{"instanceid", id}})
+        change := bson.D{{"$set", bson.D{{"instanceid", id}}}}
+        err := m.st.machines.Update(bson.D{{"_id", m.id}}, change)
         if err != nil {
                 return fmt.Errorf("can't set instance id of machine %s: %v", m, err)
         }
         return nil
 }
 
-// String returns a unique description of this machine
+// String returns a unique description of this machine.
 func (m *Machine) String() string {
         return strconv.Itoa(m.Id())
 }