go 40 lines · 8 steps

Structured validation errors in Gin

A Gin handler validates a nested JSON payload with binding tags and returns per-field errors when it fails.

Explained by highlit
Gin Intermediate validation json-binding error-handling http-handlers
1package handler
2 
3type LineItem struct {
4 SKU string `json:"sku" binding:"required,alphanum"`
5 Quantity int `json:"quantity" binding:"required,gt=0,lte=100"`
6 UnitCost float64 `json:"unit_cost" binding:"required,gt=0"`
7}
8 
9type CreateOrderRequest struct {
10 CustomerID string `json:"customer_id" binding:"required,uuid"`
11 Currency string `json:"currency" binding:"required,iso4217"`
12 Items []LineItem `json:"items" binding:"required,min=1,max=50,dive"`
13}
14 
15func CreateOrder(orders *OrderService) gin.HandlerFunc {
16 return func(c *gin.Context) {
17 var req CreateOrderRequest
18 if err := c.ShouldBindJSON(&req); err != nil {
19 var verrs validator.ValidationErrors
20 if errors.As(err, &verrs) {
21 fields := make(map[string]string, len(verrs))
22 for _, fe := range verrs {
23 fields[fe.Namespace()] = fe.Tag()
24 }
25 c.JSON(http.StatusUnprocessableEntity, gin.H{"errors": fields})
26 return
27 }
28 c.JSON(http.StatusBadRequest, gin.H{"error": "invalid request body"})
29 return
30 }
31 
32 order, err := orders.Create(c.Request.Context(), req.CustomerID, req.Currency, req.Items)
33 if err != nil {
34 c.JSON(http.StatusInternalServerError, gin.H{"error": "could not create order"})
35 return
36 }
37 
38 c.JSON(http.StatusCreated, order)
39 }
40}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Struct tags let you declare validation rules declaratively instead of writing manual checks.
  2. 2Distinguishing validation errors from other bind failures gives clients actionable, field-level feedback.
  3. 3Returning a closure over a service keeps handlers testable and free of global state.

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