Add method PrimaryKeyIsEmpty for model

model.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"reflect"
 	"regexp"
+
 	"strings"
 )
 
@@ -23,6 +24,11 @@ func (s *Orm) toModel(value interface{}) *Model {
 	return &Model{Data: value, driver: s.driver}
 }
 
+func (m *Model) PrimaryKeyIsEmpty() bool {
+	result := reflect.ValueOf(m.Data).Elem()
+	return result.FieldByName(m.PrimaryKey()).Interface().(int64) == int64(0)
+}
+
 func (m *Model) PrimaryKey() string {
 	return "Id"
 }

orm.go

@@ -3,6 +3,7 @@ package gorm
 import (
 	"database/sql"
 	"errors"
+
 	"strconv"
 )
 
@@ -86,8 +87,12 @@ func (s *Orm) Select(value interface{}) *Orm {
 }
 
 func (s *Orm) Save(value interface{}) *Orm {
-	s.explain(value, "Create").create(value)
-	// s.explain(value, "Update").update(value)
+	s.setModel(value)
+	if s.Model.PrimaryKeyIsEmpty() {
+		s.explain(value, "Create").create(value)
+	} else {
+		s.explain(value, "Update").update(value)
+	}
 	return s
 }