orm,cgen: fix @[sql: serial] and @[serial] are not the same (fix #23346) (#23678)

vlib/orm/orm_serial_attribute_test.v

@@ -1,8 +1,14 @@
 import db.sqlite
 
+struct Child {
+	id        int @[primary; serial]
+	parent_id int
+}
+
 struct PlainNoArg {
-	id   int @[primary; serial; sql: 'custom_id']
+	id       int @[primary; serial; sql: 'custom_id']
-	name string
+	name     string
+	children []Child @[fkey: 'parent_id']
 }
 
 fn test_plain_no_arg() {
@@ -33,6 +39,26 @@ fn test_plain_no_arg() {
 	assert rows[0].name == 'second'
 }
 
+fn test_plain_no_arg_with_children() {
+	mut db := sqlite.connect(':memory:')!
+
+	parent := PlainNoArg{
+		children: [Child{}, Child{}]
+	}
+
+	sql db {
+		create table Child
+		create table PlainNoArg
+		insert parent into PlainNoArg
+	}!
+
+	parents := sql db {
+		select from PlainNoArg limit 1
+	}!
+
+	assert parent.children.len == parents[0].children.len
+}
+
 struct SqlSerial {
 	id   int @[primary; sql: serial]
 	name string

vlib/v/gen/c/orm.v

@@ -331,8 +331,8 @@ fn (mut g Gen) write_orm_insert_with_last_ids(node ast.SqlStmtLine, connection_v
 
 	primary_field := g.get_orm_struct_primary_field(fields) or { ast.StructField{} }
 
-	is_serial := primary_field.attrs.contains_arg('sql', 'serial')
+	is_serial := (primary_field.attrs.contains_arg('sql', 'serial')
-		&& primary_field.typ == ast.int_type
+		|| primary_field.attrs.contains('serial')) && primary_field.typ == ast.int_type
 
 	mut inserting_object_type := ast.void_type
 	mut member_access_type := '.'