service.go

type Database interface {
  Execute(c *mgo.Collection) error
}

type Result struct{}

func MyService(db Database) (*Result, error) {
  var result Result
  
  f := func(c *mgo.Collection) error {
    return c.Find(query).One(&result)
  }
  
  if err := db.Execute(f); err != nil {
    return nil, err
  }
  
  return &result, nil
}