Golang Standard Package Structure - go

Question

I am faily new to Go and I am trying to create a structured application using guidance from Ben Johnson's webpage. Unfortunately, his example is not a complete working application.
His webpage is https://medium.com/#benbjohnson/standard-package-layout-7cdbc8391fc1
I have tried to use his methods and I keep getting "Undefined: db" error. It doesn't tell me what line is causing the error, just the file "MSSQL.go"
Could someone help with guidance to help me fix this error?
Edited code with accepted solution.
StatementPrinter.go
package statementprinter
type Statement struct {
CustomerId string
CustomerName string
}
type StatementService interface {
Statement(id string) (*Statement, error)
}
main.go
package main
import (
"fmt"
"log"
"github.com/ybenjolin/StatementPrinter"
"github.com/ybenjolin/StatementPrinter/mssql"
"database/sql"
_ "github.com/alexbrainman/odbc"
)
const DB_INFO = "Driver={SQL Server};Server=cdc-edb2;Database=CostarReports;Trusted_Connection=yes;"
var db *sql.DB
func init() {
var err error
db, err = sql.Open("odbc", DB_INFO)
if err != nil {
log.Fatal("Error opening database connection.\n", err.Error())
}
err = db.Ping()
if err != nil {
log.Fatal("Error pinging database server.\n", err.Error())
}
fmt.Println("Database connection established.")
}
func main () {
var err error
defer db.Close()
// Create services
// Changes required here. Was ss := &statementprinter.Stat..
ss := &mssql.StatementService{DB: db}
// Use service
var s *statementprinter.Statement
s, err = ss.Statement("101583")
if err != nil {
log.Fatal("Query failed:", err.Error())
}
fmt.Printf("Statement: %+v\n", s)
}
mssql.go
package mssql
import (
_ "github.com/alexbrainman/odbc"
"database/sql"
"github.com/ybenjolin/StatementPrinter"
)
// StatementService represents a MSSQL implementation of statemenetprinter.StatementService.
type StatementService struct {
DB *sql.DB
}
// Statement returns a statement for a given customer.
func (s *StatementService) Statement(customer string) (*statementprinter.Statement, error) {
var err error
var t statementprinter.Statement
// Changes required here. Was row := db.Query......
row := s.DB.QueryRow(`Select Customer, CustomerName From AccountsReceivable.rptfARStatementHeader(?)`, customer)
if row.Scan(&t.CustomerId, &t.CustomerName); err != nil {
return nil, err
}
return &t, nil

Answer 1

This seems like it's just a typo. It seems like the problematic line is in the method
func (s *StatementService) Statement(customer string)
in mssql.go,
row := db.QueryRow(`Select Customer, CustomerName From AccountsReceivable.rptfARStatementHeader(?)`, customer)
QueryRow is supposed to be a method of db, but db is not defined. However, in the struct
type StatementService struct {
DB *sql.DB
}
there's a *sql.DB instance. The method you're using has a *StatementService parameter, s. So, my guess is the intention would be to access the sql.DB field in s like so
func (s *StatementService) Statement(customer string) (*statementprinter.Statement, error) {
var err error
var t statementprinter.Statement
//CHANGED LINE:
row := s.DB.QueryRow(`Select Customer, CustomerName From AccountsReceivable.rptfARStatementHeader(?)`, customer)
if row.Scan(&t.CustomerId, &t.CustomerName); err != nil {
return nil, err
}
return &t, nil
Then, the method is called in main.go, and is passed a StatementService instance that contains a database:
ss := &statementprinter.StatementService{DB: db}
I believe you need to change this line to
ss := &mssql.StatementService{DB: db}
becuase that's the actual interface implementation. The line you have now treats the StatementService interface like a struct which will not compile.
The global db in main.go lives for the lifetime of the application. It's just a pointer which is copied around for use.