Add exists tag

exists tag used to ensure that a Pointer, Interface or
Invalid has a value, but won't interfere with any other validation.

doc.go

@@ -102,6 +102,14 @@ Here is a list of the current built in validators:
 		you know the struct will be valid, but need to verify it has been assigned.
 		NOTE: only "required" and "omitempty" can be used on a struct itself.
 
+	exists
+		Is a special tag without a validation function attached. It is used when a field
+		is a Pointer, Interface or Invalid and you wish to validate that it exists.
+		Example: want to ensure a bool exists if you define the bool as a pointer and
+		use exists it will ensure there is a value; couldn't use required as it would
+		fail when the bool was false. exists will fail is the value is a Pointer, Interface
+		or Invalid and is nil. (Usage: exists)
+
 	omitempty
 		Allows conditional validation, for example if a field is not set with
 		a value (Determined by the "required" validator) then other validation

validator.go

@@ -29,6 +29,7 @@ const (
 	omitempty           = "omitempty"
 	skipValidationTag   = "-"
 	diveTag             = "dive"
+	existsTag           = "exists"
 	fieldErrMsg         = "Key: \"%s\" Error:Field validation for \"%s\" failed on the \"%s\" tag"
 	arrayIndexFieldName = "%s[%d]"
 	mapIndexFieldName   = "%s[%v]"
@@ -418,6 +419,10 @@ func (v *Validate) traverseField(topStruct reflect.Value, currentStruct reflect.
 
 	for _, cTag := range tags {
 
+		if cTag.tagVals[0][0] == existsTag {
+			continue
+		}
+
 		if cTag.tagVals[0][0] == diveTag {
 			dive = true
 			diveSubTag = strings.TrimLeft(strings.SplitN(tag, diveTag, 2)[1], ",")

validator_test.go

@@ -3,6 +3,7 @@ package validator
 import (
 	"database/sql"
 	"database/sql/driver"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"reflect"
@@ -191,6 +192,36 @@ func ValidateValuerType(field reflect.Value) interface{} {
 	return nil
 }
 
+func TestExistsValidation(t *testing.T) {
+
+	jsonText := "{ \"truthiness2\": true }"
+
+	type Thing struct {
+		Truthiness *bool `json:"truthiness" validate:"exists,required"`
+	}
+
+	var ting Thing
+
+	err := json.Unmarshal([]byte(jsonText), &ting)
+	Equal(t, err, nil)
+	NotEqual(t, ting, nil)
+	Equal(t, ting.Truthiness, nil)
+
+	errs := validate.Struct(ting)
+	NotEqual(t, errs, nil)
+	AssertError(t, errs, "Thing.Truthiness", "Truthiness", "exists")
+
+	jsonText = "{ \"truthiness\": true }"
+
+	err = json.Unmarshal([]byte(jsonText), &ting)
+	Equal(t, err, nil)
+	NotEqual(t, ting, nil)
+	Equal(t, ting.Truthiness, true)
+
+	errs = validate.Struct(ting)
+	Equal(t, errs, nil)
+}
+
 func TestSQLValue2Validation(t *testing.T) {
 
 	config := Config{