modify behavior for auto_increment column

README.md

@@ -36,6 +36,19 @@ Basically, inserting struct values are automatically chosen. However if you want
 
 In the above pattern `Name` and `Email` fields are excluded.
 
+### AUTO INCREMENT
+
+If auto_increment attribute is assigned to the primary key etc, make sure to set `AUTO_INCREMENT` in model struct.
+
+```go
+type fakeTable struct {
+	ID        int `gorm:"AUTO_INCREMENT"` 
+	Name      string
+}
+```
+
+This is necessary to exclude the auto_increment column from insertion and be numbered by database system. The reason for this requirement is that if all primary keys are excluded, only the surrogate key like auto_increment can be inserted and natural key cannot at all.
+
 ### Feature
 
 - Just pass a slice of struct as using gorm normally, records will be created.

bulk_insert.go

@@ -103,7 +103,7 @@ func extractMapValue(value interface{}, excludeColumns []string) (map[string]int
 		_, hasForeignKey := field.TagSettingsGet("FOREIGNKEY")
 
 		if !containString(excludeColumns, field.Struct.Name) && field.StructField.Relationship == nil && !hasForeignKey &&
-			!field.IsIgnored && !(field.DBName == "id" || fieldIsAutoIncrement(field)) {
+			!field.IsIgnored && !fieldIsAutoIncrement(field) {
 			if field.Struct.Name == "CreatedAt" || field.Struct.Name == "UpdatedAt" {
 				attrs[field.DBName] = time.Now()
 			} else if field.StructField.HasDefaultValue && field.IsBlank {
@@ -125,5 +125,5 @@ func fieldIsAutoIncrement(field *gorm.Field) bool {
 	if value, ok := field.TagSettingsGet("AUTO_INCREMENT"); ok {
 		return strings.ToLower(value) != "false"
 	}
-	return field.IsPrimaryKey
+	return false
 }