How to Customize Error Messages from Gin's Struct Tag Validation?

Customizing Error Messages from Struct Tag Validation

In Gin, struct tag validation is used to ensure data integrity before database insertion. The default error returned is verbose and uninformative for the user. This article describes how to customize the error message to provide a more meaningful response.

Gin utilizes the go-playground/validator/v10 package for validation. Errors are returned as validator.ValidationErrors. To customize the message, use the standard errors package to unwrap the error and access the validator.FieldError fields.

Create an error model (e.g., ApiError) to define the structure of your custom error message. For instance:

type ApiError struct {
    Field string
    Msg   string
}

In your handler, handle the error as follows:

var u User
err := c.BindQuery(&u);
if err != nil {
    var ve validator.ValidationErrors
    if errors.As(err, &ve) {
        out := make([]ApiError, len(ve))
        for i, fe := range ve {
            out[i] = ApiError{fe.Field(), msgForTag(fe.Tag())}
        }
        c.JSON(http.StatusBadRequest, gin.H{"errors": out})
    }
    return
}

Finally, define a helper function to map validation tags to custom messages:

func msgForTag(tag string) string {
    switch tag {
    case "required":
        return "This field is required"
    case "email":
        return "Invalid email"
    }
    return ""
}

Using this approach, error messages will be customized with the field name and a tailored message based on the validation tag.

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