Erro ao gravar carrinho banco de dados com hibernate

Boa tarde a todos, estou com esse erro no sistema, alguém pode me ajudar?

Grave: org.hibernate.exception.DataException: could not insert: [br.com.vendas.modelo.Venda]
javax.faces.el.EvaluationException: org.hibernate.exception.DataException: could not insert: [br.com.vendas.modelo.Venda]
at javax.faces.component.MethodBindingMethodExpressionAdapter.invoke(MethodBindingMethodExpressionAdapter.java:102)
at com.sun.faces.application.ActionListenerImpl.processAction(ActionListenerImpl.java:102)
at javax.faces.component.UICommand.broadcast(UICommand.java:315)
at javax.faces.component.UIViewRoot.broadcastEvents(UIViewRoot.java:775)
at javax.faces.component.UIViewRoot.processApplication(UIViewRoot.java:1267)

att,
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