handler.go

func LoginFn(c *gin.Context) {
	user := models.User{}
	session := sessions.Default(c)
	u := session.Get("user")
	if u != nil {
		user = u.(models.User)
	}

	if u != nil && user.IsAuthenticated {
		// There is no need to see login page
		// Showing index
		c.HTML(http.StatusOK, "index", gin.H{
			"title": "Hi! GoBlog.",
			"user":  user,
		})
		return
	}

	// The user is not authenticated
	if c.Request.Method == http.MethodGet { // Serve login page
		c.HTML(http.StatusOK, "login", gin.H{
			"title": "Hi! GoBlog.",
		})
	} else if c.Request.Method == http.MethodPost { // Process login
		dbc, exist := c.Get("db")
		if !exist {
			c.HTML(http.StatusOK, "login", gin.H{
				"title": "Hi! GoBlog.",
				"error": "We've got an internal error, please try later.",
			})
			return
		}

		var login models.LoginForm
		db := dbc.(*gorm.DB)
		if err := c.ShouldBind(&login); err == nil {
			db.Where(&models.User{Username: login.Username}).First(&user)
			if user.CheckPassword(login.Password) {
				user.IsAuthenticated = true
				session.Set("user", user)
				if err = session.Save(); err != nil {
					c.HTML(http.StatusOK, "login", gin.H{
						"title": "Hi! GoBlog.",
						"error": "Internal error. Please try later",
					})
					return
				}
				c.HTML(http.StatusOK, "index", gin.H{
					"title": "Hi! GoBlog.",
					"user":  user,
				})
			} else {
				c.HTML(http.StatusOK, "login", gin.H{
					"title": "Hi! GoBlog.",
					"error": "Username or Password incorrect, please try again.",
				})
			}
		} else {
			c.HTML(http.StatusOK, "login", gin.H{
				"title": "Hi! GoBlog.",
				"error": "Required fields are not provided correctly.",
			})
		}
	}
}