code: func (q *Query) Select(selector interface{}) *Query {
  	q.m.Lock()
  	q.op.selector = selector
  	q.m.Unlock()
  	return q
  }
docs: // Select enables selecting which fields should be retrieved for the results
  // found. For example, the following query would only retrieve the name field:
  //
  //     err := collection.Find(nil).Select(bson.M{"name": 1}).One(&result)
  //
  // Relevant documentation:
  //
  //     http://www.mongodb.org/display/DOCS/Retrieving+a+Subset+of+Fields
  //