combining GO GIN-GONIC GORM and VALIDATOR.V2

I am quite new to Go and I would like to startup by setting a GIN-GONIC API. I found this tutorial and I am very happy with that skeleton. But now I am stuck with the validating process which I added: "gopkg.in/validator.v2" and

type Todo struct {
    gorm.Model
	Title     string `json:"title"`
    Completed int `json:"completed"`
}

became

type Todo struct {
    gorm.Model
	Title     string `json:"title" **validate:"size:2"**`
    Completed int `json:"completed"`
}

and then in the CreateTodo function which I added :

if errs := validator.Validate(todo); errs!=nil {
	c.JSON(500, gin.H{"Error": errs.Error()})
}

but then a POST call send :

> "Error": "Type: unknown tag"

after some research I found that :
> Using a non-existing validation func in a field tag will always return false and with error validate.ErrUnknownTag.

so the **validate:"size:2"** must be wrong ...

I don't get how to set the validation and also how to display the correct error within the "catch":

c.JSON(500, gin.H{"Error": errs.Error()})

Looks like you haven't defined size validation function. Also you can do it.

Custom validation functions:

func size(v interface{}, param int) error {
    st := reflect.ValueOf(v)
	if st.Kind() != reflect.String {
    	return validate.ErrUnsupported
	}
    if utf8.RuneCountInString(st.String()) != param {
	    return errors.New("Wrong size")
	}
    return nil
}
validate.SetValidationFunc("size", size)