How do I populate a drop down with a list using thymeleaf and spring

[solved] caused by: org.hibernate.PersistentObjectException: detached entity passed to persist:

The error looks like org.hibernate.PersistentObjectException: detached entity passed to persist: example.forms.InvoiceItem         at org.hibernate.event.def.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:127)     at org.hibernate.impl.SessionImpl.firePersist(SessionImpl.java:799)         at org.hibernate.impl.SessionImpl.persist(SessionImpl.java:791)      …