【JPA】JPA 多对多映射报错信息：detached entity passed to persist原因汇总

org.springframework.dao.InvalidDataAccessApiUsageException: detached entity passed to persist: com.wxl.pojo.Tag; nested exception is org.hibernate.PersistentObjectException: detached entity passed to persist: com.wxl.pojo.Tag
	at org.springframework.orm.jpa.vendor.HibernateJpaDialect.convertHibernateAccessException(HibernateJpaDialect.java:299)
	at org.springframework.orm.jpa.vendor.HibernateJpaDialect.translateExceptionIfPossible(HibernateJpaDialect.java:244)
	at org.springframework.orm.jpa.AbstractEntityManagerFactoryBean.translateExceptionIfPossible(AbstractEntityManagerFactoryBean.java:488)
	at org.springframework.dao.support.ChainedPersistenceExceptionTranslator.translateExceptionIfPossible(ChainedPersistenceExceptionTranslator.java:59)
	at org.springframework.dao.support.DataAccessUtils.translateIfNecessary(DataAccessUtils.java:213)
	at org.springframework.dao.support.PersistenceExceptionTranslationInterceptor.invoke(PersistenceExceptionTranslationInterceptor.java:147)
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
	at org.springframework.data.jpa.repository.support.CrudMethodMetadataPostProcessor$CrudMethodMetadataPopulatingMethodInterceptor.invoke(CrudMethodMetadataPostProcessor.java:133)
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
	at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:92)
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
	at org.springframework.data.repository.core.support.SurroundingTransactionDetectorMethodInterceptor.invoke(Surroun

原因一：

保存时预先设置一个任意主键：

blog.setId((long) 0);

原因二

比如:在实体类中定义的变量如下:
@Id
@GeneratedValue
private Long id;//主键

系统默认这个值是由数据库自动生成的,可是你的数据库并没设置其为identity属性.
这样就会报错.

解决办法1.在存储数据的时候,要记得把id这一项设置为空.否则,是不可以存储的,因为已经identity了.

解决办法2.去掉 @GeneratedValue()

原因三

将 @ManyToMany 的 cascade 属性设置为 Cascade.MERGE 或者 Cascade.REFRESH ，慎用 Cascade.ALL