Try to add support for NullInt64

chain.go

@@ -70,6 +70,7 @@ func (s *Chain) do(value interface{}) *Do {
 	do.specifiedTableName = s.specifiedTableName
 	do.unscoped = s.unscoped
 	do.singularTableName = s.singularTableName
+	do.debug = s.debug
 
 	s.value = value
 	do.setModel(value)

do.go

@@ -17,6 +17,7 @@ type Do struct {
 	driver             string
 	guessedTableName   string
 	specifiedTableName string
+	debug              bool
 	Errors             []error
 
 	model     *Model

gorm_test.go

@@ -1081,7 +1081,7 @@ type Category struct {
 
 type Post struct {
 	Id             int64
-	CategoryId     int64
+	CategoryId     sql.NullInt64
 	MainCategoryId int64
 	Title          string
 	Body           string
@@ -1124,7 +1124,7 @@ func TestSubStruct(t *testing.T) {
 
 	var p Post
 	db.First(&p, post.Id)
-	if post.CategoryId == 0 || p.CategoryId == 0 || post.MainCategoryId == 0 || p.MainCategoryId == 0 {
+	if post.CategoryId.Int64 == 0 || p.CategoryId.Int64 == 0 || post.MainCategoryId == 0 || p.MainCategoryId == 0 {
 		t.Errorf("Category Id should exist")
 	}
 

model.go

@@ -1,6 +1,7 @@
 package gorm
 
 import (
+	"database/sql"
 	"errors"
 	"fmt"
 	"go/ast"
@@ -153,6 +154,9 @@ func (m *Model) fields(operation string) (fields []Field) {
 					if is_time {
 						field.SqlType = getSqlType(m.driver, field.Value, 0)
 					} else {
+						switch value.Interface().(type) {
+						case sql.NullInt64, sql.NullFloat64, sql.NullBool, sql.NullString:
+						default:
 							if indirect_value.FieldByName(p.Name + "Id").IsValid() {
 								field.foreignKey = p.Name + "Id"
 								field.beforeAssociation = true
@@ -164,6 +168,7 @@ func (m *Model) fields(operation string) (fields []Field) {
 								field.afterAssociation = true
 							}
 						}
+					}
 				case reflect.Ptr:
 					debug("Errors when handle ptr sub structs")
 				default:
@@ -363,7 +368,22 @@ func setFieldValue(field reflect.Value, value interface{}) bool {
 			}
 			field.SetInt(reflect.ValueOf(value).Int())
 		default:
+			field_type := field.Type()
+			if field_type == reflect.TypeOf(value) {
 				field.Set(reflect.ValueOf(value))
+			} else if value == nil {
+				field.Set(reflect.Zero(field.Type()))
+			} else if field_type == reflect.TypeOf(sql.NullBool{}) {
+				field.Set(reflect.ValueOf(sql.NullBool{value.(bool), true}))
+			} else if field_type == reflect.TypeOf(sql.NullFloat64{}) {
+				field.Set(reflect.ValueOf(sql.NullFloat64{value.(float64), true}))
+			} else if field_type == reflect.TypeOf(sql.NullInt64{}) {
+				field.Set(reflect.ValueOf(sql.NullInt64{value.(int64), true}))
+			} else if field_type == reflect.TypeOf(sql.NullString{}) {
+				field.Set(reflect.ValueOf(sql.NullString{value.(string), true}))
+			} else {
+				field.Set(reflect.ValueOf(value))
+			}
 		}
 		return true
 	} else {