code: func (c *DialContext) EnsureUniqueIndex(db string, collection string, key []string) error {
  	s := c.Ref()
  	defer c.UnRef(s)
  
  	return s.DB(db).C(collection).EnsureIndex(mgo.Index{
  		Key:    key,
  		Unique: true,
  		Sparse: true,
  	})
  }
docs: // goroutine safe