Config package with global variable

_tree

.
├── config
│   └── config.go
├── go.mod
├── go.sum
├── main.go
└── models
    └── books
        └── books.go

books.go

package books

import (
	"bookstore.alexedwards.net/config"
)

type Book struct {
	Isbn   string
	Title  string
	Author string
	Price  float32
}

func All() ([]Book, error) {
	rows, err := config.DB.Query("SELECT * FROM books")
	if err != nil {
		return nil, err
	}
	defer rows.Close()

	var bks []Book

	for rows.Next() {
		var bk Book

		err := rows.Scan(&bk.Isbn, &bk.Title, &bk.Author, &bk.Price)
		if err != nil {
			return nil, err
		}

		bks = append(bks, bk)
	}
	if err = rows.Err(); err != nil {
		return nil, err
	}

	return bks, nil
}

config.go

package config

import (
	"database/sql"
)

var DB *sql.DB

go.mod

module bookstore.alexedwards.net

go 1.15

require github.com/lib/pq v1.8.0

main.go

package main

import (
	"database/sql"
	"fmt"
	"log"
	"net/http"

	"bookstore.alexedwards.net/config"
	"bookstore.alexedwards.net/models/books"

	_ "github.com/lib/pq"
)

func main() {
	var err error

	config.DB, err = sql.Open("postgres", "postgres://user:pass@localhost/bookstore")
	if err != nil {
		log.Fatal(err)
	}

	http.HandleFunc("/books", booksIndex)
	http.ListenAndServe(":3000", nil)
}

func booksIndex(w http.ResponseWriter, r *http.Request) {
	bks, err := books.All()
	if err != nil {
		log.Println(err)
		http.Error(w, http.StatusText(500), 500)
		return
	}

	for _, bk := range bks {
		fmt.Fprintf(w, "%s, %s, %s, £%.2f\n", bk.Isbn, bk.Title, bk.Author, bk.Price)
	}
}