Preparing a PG DB and Go backend
Lets Create Bookstore Database where we fetch book details
Creating a BookStore DB in PostgreSQL
I'll start with the installation
$ sudo apt install postgresql
$ sudo -u postgres psql
Logging into postgres
psql -h localhost -U postgres
Postgres console will be activated
psql (14.11 (Ubuntu 14.11-0ubuntu0.22.04.1))
Type "help" for help.
postgres=#
Create DB name bookstore by this command
postgres=CREATE DATABASE bookstore;
CREATE DATABASE
Check out the DB and create Table
postgres=# \c bookstore
SSL connection (protocol: TLSv1.3, cipher: TLS_AES_256_GCM_SHA384, bits: 256, compression: off)
You are now connected to the database "bookstore" as user "postgres".
Create a Table in the bookstore DB
CREATE TABLE books (
isbn char(14) NOT NULL,
title varchar(255) NOT NULL,
author varchar(255) NOT NULL,
price decimal(5,2) NOT NULL
);
INSERT INTO books (isbn, title, author, price) VALUES
('9780099470464', 'A House for Mr. Biswas', 'V. S. Naipaul', 8.99),
('9780143037346', 'Miss New India', 'Bharati Mukherjee', 9.99),
('9781784782781', 'The Lives of Others', 'Neel Mukherjee', 11.99),
ALTER TABLE books ADD PRIMARY KEY (isbn);
Then Verify whether it is created
bookstore-# \dt
List of relations
Schema | Name | Type | Owner
--------+-------+-------+----------
public | books | table | postgres
(1 row)
Setting up Go backend
$ mkdir bookstore && cd bookstore
$ mkdir models
$ touch main.go models/models.go
$ go mod init go.backend
go: creating new go.mod: module go.backend
File Structure of example :
bookstore/
├── go.mod
├── go.sum
├── main.go
└── models
└── models.go
Set a Global DB Instance
This method simplifies accessing the database connection across the application.
It initializes the database connection, sets up an HTTP server, and listens for incoming requests.
In the context of our bookstore application, the code would look something like this
// models/models.go
package models
import (
"database/sql"
)
Create an exported global variable to hold the database connection pool.
// models/models.go
var DB *sql.DB
type Book struct {
Isbn string
Title string
Author string
Price float32
}
AllBooks() returns a slice of all books in the books table.
// models/models.go
func AllBooks() ([]Book, error) {
// Note that we are calling Query() on the global variable.
rows, err := 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
}
// main.go
package main
import (
"database/sql"
"fmt"
"log"
"net/http"
"go.backend/models"
_ "github.com/lib/pq"
)
Install dependencies
$ go get github.com/lib/pq
Initialize the sql.DB connection pool and assign it to the models.DB
func main() {
var err error
// global variable.
models.DB, err = sql.Open("postgres", "postgres://user:pass@localhost/bookstore")
if err != nil {
log.Fatal(err)
}
http.HandleFunc("/books", booksIndex)
http.ListenAndServe(":3000", nil)
}
When I make a request books
$ curl localhost:3000/books
978-1503261969, Emma, Jayne Austen, £9.44
978-1505255607, The Time Machine, H. G. Wells, £5.99
978-1503379640, The Prince, Niccolò Machiavelli, £6.99
Remember global variable for the database connection is suitable when:
- Your project is simple and small, so tracking global variables isn't hard.
- Your code handling web requests is split across different folders, but all database actions stay in one folder.
- You don't need to pretend the database isn't there for testing.
Global variable with an InitDB function
A variation on the 'global variable' approach that I sometimes see uses an initialization function to set up the connection pool, like so:
- All database stuff is in one place.
- The global database variable is hidden from other parts of the program, so it can't be changed by mistake.
- During testing, you can easily set up a test database connection using a special function.
// models/models.go
package models
import (
"database/sql"
_ "github.com/lib/pq"
)
Initialize the sql.DB connection pool and assign it to the models.DB
// This time the global variable is unexported.
var db *sql.DB
// InitDB sets up setting up the connection pool global variable.
func InitDB(dataSourceName string) error {
var err error
db, err = sql.Open("postgres", dataSourceName)
if err != nil {
return err
}
return db.Ping()
}
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