mcloud | 基于Spring Cloud，实现微服务中常用的基础模块，包括 OAuth2 认证服务，统一注册中心，系统监控中心， | Microservice library

First you need to tell encoding/json to skip the parent field, you can do that with the json:"-" tag.

Then during unmarshaling, after you've loaded all of a parent's children you loop over the children and set their parent, you can do this as part of the unamrshaling process by implementing the json.Unmarshaler interface.

you test work inside one transaction ,in this you work with (u2.getCreditCardCollection()) with values from object not with entity (save is not executed yet):

Assert.assertEquals("User should have 2 credit cards.", 1, u2.getCreditCardCollection().size());

when you iterate over u2.getCreditCardCollection() and check id , they are null as insert and id generation comes only in commit stage or flush/clear context.

You need call saveAndFlush() instead of just save().