Custom Validators

It is also possible to register custom validators. See the example code.

package main
import (
    "net/http"
    "reflect"
    "time"
    "github.com/gin-gonic/gin"
    "github.com/gin-gonic/gin/binding"
    "gopkg.in/go-playground/validator.v8"
)
type Booking struct {
    CheckIn  time.Time `form:"check_in" binding:"required,bookabledate" time_format:"2006-01-02"`
    CheckOut time.Time `form:"check_out" binding:"required,gtfield=CheckIn" time_format:"2006-01-02"`
}
func bookableDate(
    v *validator.Validate, topStruct reflect.Value, currentStructOrField reflect.Value,
    field reflect.Value, fieldType reflect.Type, fieldKind reflect.Kind, param string,
) bool {
    if date, ok := field.Interface().(time.Time); ok {
        today := time.Now()
        if today.Year() > date.Year() || today.YearDay() > date.YearDay() {
            return false
        }
    }
    return true
}
func main() {
    route := gin.Default()
    if v, ok := binding.Validator.Engine().(*validator.Validate); ok {
        v.RegisterValidation("bookabledate", bookableDate)
    }
    route.GET("/bookable", getBookable)
    route.Run(":8085")
}
func getBookable(c *gin.Context) {
    var b Booking
    if err := c.ShouldBindWith(&b, binding.Query); err == nil {
        c.JSON(http.StatusOK, gin.H{"message": "Booking dates are valid!"})
    } else {
        c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
    }
}

$ curl "localhost:8085/bookable?check_in=2018-04-16&check_out=2018-04-17"
{"message":"Booking dates are valid!"}
$ curl "localhost:8085/bookable?check_in=2018-03-08&check_out=2018-03-09"
{"error":"Key: 'Booking.CheckIn' Error:Field validation for 'CheckIn' failed on the 'bookabledate' tag"}

Struct level validations can also be registed this way.
See the struct-lvl-validation example to learn more.