Godeps/_workspace/src/gopkg.in/mgo.v2/session.go
func (p *Pipe) One(result interface{}) error {
	iter := p.Iter()
	if iter.Next(result) {
		return nil
	}
	if err := iter.Err(); err != nil {
		return err
	}
	return ErrNotFound
}
func (p *Pipe) One(result interface{}) error {
	iter := p.Iter()
	if iter.Next(result) {
		return nil
	}
	if err := iter.Err(); err != nil {
		return err
	}
	return ErrNotFound
}
// One executes the pipeline and unmarshals the first item from the
// result set into the result parameter.
// It returns ErrNotFound if no items are generated by the pipeline.