Golang : Pass database connection to function called from another package and HTTP Handler

For this tutorial, we will explore how to pass database connection to external function called from another package. To pass a database connection is simple, just treat the database connection as a variable and at the function located in package, use pointer to point to the database connection variable. This tutorial will also cover how to pass database connection to HTTP Handler function as well.

Here we go!

 package main

 import (
  "database/sql"
  "fmt"

  // without the underscore _, you will get imported but not
  // used error message
  _ "github.com/go-sql-driver/mysql"
  "mypackage/myfunction"
  "os"
 )

 func main() {

  conn, err := sql.Open("mysql", "user:password@tcp(127.0.0.1:3306)/database")
  if err != nil {
 fmt.Println(err)
 os.Exit(1)
  }

  // Pass conn (sql.DB type variable) to external function
  myfunction.PrintTitles(conn)

  conn.Close()
 }

and then in the external function, remember to use *sql.DB

 package myfunction

 import (
  "database/sql"
  "fmt"
  _ "github.com/go-sql-driver/mysql"
  "os"
 )

 func PrintTitles(conn *sql.DB) {

  // use your own select statement
  // this is just an example statement
  statement, err := conn.Prepare("select blogtitle from blogs limit 10")

  if err != nil {
 fmt.Println(err)
 os.Exit(1)
  }

  rows, err := statement.Query() // execute our select statement

  if err != nil {
 fmt.Println(err)
 os.Exit(1)
  }

  for rows.Next() {
 var blogtitle string
 rows.Scan(&blogtitle)
 fmt.Println("Title :", blogtitle)
  }
 }

and if you are calling your external function with HTTP Handler, such as :

  mx := mux.NewRouter()
  mx.HandleFunc("/", PrintTitles(conn))

then you need to wrap your function with a closure. So, instead of :

  func PrintTitles(conn *sql.DB) (w http.ResponseWriter, r *http.Request) {
 ...
  }

wrap(close) it with a anonymous function

 func PrintTitles(conn *sql.DB) func(w http.ResponseWriter, r *http.Request) {
 return func(w http.ResponseWriter, r *http.Request) {
 ...
 // can access conn variable now
 }
  }

Happy coding!

References :

https://www.socketloop.com/tutorials/golang-call-function-from-another-package

https://www.socketloop.com/tutorials/golang-connect-to-database-mysql-mariadb-server