sour grapes idiom
酸葡萄（指由于得不到而贬低某物）
If you describe someone's behaviour or opinion as sour grapes, you mean that that person is angry because they have not got or achieved something that they wanted.
I don't think it's such a great job - and that's not just sour grapes because I didn't get it. 我认为这份工作不怎么样——我可不是因为吃不着葡萄说葡萄酸。