Validation in Go Gin: Binding Request Data (JSON, Form, Query)

In Go Gin, data binding (extracting request data into a struct) and validation are handled using struct tags. Gin supports binding and validating data from JSON, forms, query parameters, etc.

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🔹 1. Binding JSON Data

Gin provides methods to bind request data to structs:

• c.ShouldBind() or c.ShouldBindJSON(): Returns an error if data is invalid.
• c.MustBind() or c.MustBindJSON(): Automatically returns HTTP 400 if an error occurs.

Example with JSON request:

go
package main

import (
	"github.com/gin-gonic/gin"
	"net/http"
)

type User struct {
	Name  string `json:"name" binding:"required"`
	Email string `json:"email" binding:"required,email"`
	Age   int    `json:"age" binding:"gte=18,lte=60"`
}

func main() {
	r := gin.Default()

	r.POST("/register", func(c *gin.Context) {
		var user User

		// Bind JSON request and validate
		if err := c.ShouldBindJSON(&user); err != nil {
			c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
			return
		}

		c.JSON(http.StatusOK, gin.H{"message": "Register success!", "user": user})
	})

	r.Run(":8080")
}

📌 Explanation:

• binding:"required" → Field is mandatory.
• binding:"email" → Must be a valid email.
• binding:"gte=18,lte=60" → Age must be between 18 and 60.

---

🔹 2. Binding Form Data (x-www-form-urlencoded)

For form-data requests, use ShouldBind() or ShouldBindForm():

go
type LoginForm struct {
	Username string `form:"username" binding:"required"`
	Password string `form:"password" binding:"required,min=6"`
}

r.POST("/login", func(c *gin.Context) {
	var form LoginForm
	if err := c.ShouldBind(&form); err != nil {
		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
		return
	}

	c.JSON(http.StatusOK, gin.H{"message": "Login success!", "user": form})
})

📌 Notes:

• form:"username" → Extracts data from form fields.
• binding:"min=6" → Password must have at least 6 characters.

---

🔹 3. Binding Query Parameters

If data comes from query parameters, use ShouldBindQuery():

go
type QueryParams struct {
	Page  int `form:"page" binding:"required,gte=1"`
	Limit int `form:"limit" binding:"required,gte=5,lte=50"`
}

r.GET("/search", func(c *gin.Context) {
	var query QueryParams
	if err := c.ShouldBindQuery(&query); err != nil {
		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
		return
	}

	c.JSON(http.StatusOK, gin.H{"page": query.Page, "limit": query.Limit})
})

📌 Query Example:
GET /search?page=1&limit=10

---

🔹 4. Custom Validation

For custom validation rules, use validator.v10:

go
import (
	"github.com/gin-gonic/gin"
	"github.com/go-playground/validator/v10"
	"net/http"
)

type User struct {
	Username string `json:"username" binding:"required,alphanum"`
	Age      int    `json:"age" binding:"checkAge"`
}

// Custom rule: age must be 18+
var validate *validator.Validate

func checkAge(fl validator.FieldLevel) bool {
	return fl.Field().Int() >= 18
}

func main() {
	r := gin.Default()
	validate = validator.New()
	validate.RegisterValidation("checkAge", checkAge)

	r.POST("/custom", func(c *gin.Context) {
		var user User
		if err := c.ShouldBindJSON(&user); err != nil {
			c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
			return
		}
		c.JSON(http.StatusOK, gin.H{"message": "User valid!", "user": user})
	})

	r.Run(":8080")
}

📌 Key Features:

• RegisterValidation("checkAge", checkAge) → Registers custom validation.
• binding:"alphanum" → Ensures the username contains only letters and numbers.
• checkAge function → Prevents users under 18 years old.

---

🔹 Summary

Method	Data Source
ShouldBindJSON(&obj)	JSON body
ShouldBind(&obj)	Auto-detects (JSON, form, query)
ShouldBindQuery(&obj)	Query parameters
ShouldBindForm(&obj)	Form-data

✅ Using validation in Gin ensures data integrity, improves security, and simplifies API development! 🚀