detached entity passed to persist JBoss7

I'm trying to make a project (home work) which takes informations from a web app and store them into a database using an EJB. (these are my first steps btw). I'm getting an exception that i couldn't solve. I'm using JBoss7.Here is the exception:

 ERROR [org.jboss.ejb3.invocation] (http-localhost-127.0.0.1-8180-1) JBAS014134: EJB Invocation failed on component ENIT for method public abstract void metier.sessions.EnitLocal.addCours(metier.entities.Cours): javax.ejb.EJBException: javax.persistence.PersistenceException: org.hibernate.PersistentObjectException: detached entity passed to persist: metier.entities.Cours
at org.jboss.as.ejb3.tx.CMTTxInterceptor.handleExceptionInOurTx(CMTTxInterceptor.java:166) [jboss-as-ejb3-7.1.0.Final.jar:7.1.0.Final]
at org.jboss.as.ejb3.tx.CMTTxInterceptor.invokeInOurTx(CMTTxInterceptor.java:230) [jboss-as-ejb3-7.1.0.Final.jar:7.1.0.Final]
at org.jboss.as.ejb3.tx.CMTTxInterceptor.required(CMTTxInterceptor.java:304) [jboss-as-ejb3-7.1.0.Final.jar:7.1.0.Final]
at org.jboss.as.ejb3.tx.CMTTxInterceptor.processInvocation(CMTTxInterceptor.java:190) [jboss-as-ejb3-7.1.0.Final.jar:7.1.0.Final]
at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288) [jboss-invocation-1.1.1.Final.jar:1.1.1.Final]
at org.jboss.as.ejb3.component.interceptors.CurrentInvocationContextInterceptor.processInvocation(CurrentInvocationContextInterceptor.java:41) [jboss-as-ejb3-7.1.0.Final.jar:7.1.0.Final]
at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288) [jboss-invocation-1.1.1.Final.jar:1.1.1.Final]
at org.jboss.as.ejb3.component.interceptors.LoggingInterceptor.processInvocation(LoggingInterceptor.java:59) [jboss-as-ejb3-7.1.0.Final.jar:7.1.0.Final]
at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288) [jboss-invocation-1.1.1.Final.jar:1.1.1.Final]
at org.jboss.as.ee.component.NamespaceContextInterceptor.processInvocation(NamespaceContextInterceptor.java:50) [jboss-as-ee-7.1.0.Final.jar:7.1.0.Final]
at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288) [jboss-invocation-1.1.1.Final.jar:1.1.1.Final]
at org.jboss.as.ee.component.TCCLInterceptor.processInvocation(TCCLInterceptor.java:45) [jboss-as-ee-7.1.0.Final.jar:7.1.0.Final]
at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288) [jboss-invocation-1.1.1.Final.jar:1.1.1.Final]
at org.jboss.invocation.ChainedInterceptor.processInvocation(ChainedInterceptor.java:61) [jboss-invocation-1.1.1.Final.jar:1.1.1.Final]
at org.jboss.as.ee.component.ViewService$View.invoke(ViewService.java:165) [jboss-as-ee-7.1.0.Final.jar:7.1.0.Final]
at org.jboss.as.ee.component.ViewDescription$1.processInvocation(ViewDescription.java:173) [jboss-as-ee-7.1.0.Final.jar:7.1.0.Final]
at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288) [jboss-invocation-1.1.1.Final.jar:1.1.1.Final]
at org.jboss.invocation.ChainedInterceptor.processInvocation(ChainedInterceptor.java:61) [jboss-invocation-1.1.1.Final.jar:1.1.1.Final]
at org.jboss.as.ee.component.ProxyInvocationHandler.invoke(ProxyInvocationHandler.java:72) [jboss-as-ee-7.1.0.Final.jar:7.1.0.Final]
at metier.sessions.EnitLocal$$$view1.addCours(Unknown Source) [EnitEJB.jar:]
at Servlets.Controleur.doPost(Controleur.java:52)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:754) [jboss-servlet-api_3.0_spec-1.0.0.Final.jar:1.0.0.Final]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:847) [jboss-servlet-api_3.0_spec-1.0.0.Final.jar:1.0.0.Final]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:329)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:248)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:275)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:161)
at org.jboss.as.jpa.interceptor.WebNonTxEmCloserValve.invoke(WebNonTxEmCloserValve.java:50) [jboss-as-jpa-7.1.0.Final.jar:7.1.0.Final]
at org.jboss.as.web.security.SecurityContextAssociationValve.invoke(SecurityContextAssociationValve.java:154)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:155)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:368)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:877)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:671)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:930)
at java.lang.Thread.run(Unknown Source) [rt.jar:1.7.0_45]

Caused by: javax.persistence.PersistenceException: org.hibernate.PersistentObjectException: detached entity passed to persist: metier.entities.Cours at org.hibernate.ejb.AbstractEntityManagerImpl.convert(AbstractEntityManagerImpl.java:1361) [hibernate-entitymanager-4.0.1.Final.jar:4.0.1.Final] at org.hibernate.ejb.AbstractEntityManagerImpl.convert(AbstractEntityManagerImpl.java:1289) [hibernate-entitymanager-4.0.1.Final.jar:4.0.1.Final] at org.hibernate.ejb.AbstractEntityManagerImpl.convert(AbstractEntityManagerImpl.java:1295) [hibernate-entitymanager-4.0.1.Final.jar:4.0.1.Final] at org.hibernate.ejb.AbstractEntityManagerImpl.persist(AbstractEntityManagerImpl.java:859) [hibernate-entitymanager-4.0.1.Final.jar:4.0.1.Final] at org.jboss.as.jpa.container.AbstractEntityManager.persist(AbstractEntityManager.java:563) [jboss-as-jpa-7.1.0.Final.jar:7.1.0.Final] at metier.sessions.EnitImpl.addCours(EnitImpl.java:29) [EnitEJB.jar:] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) [rt.jar:1.7.0_45] at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) [rt.jar:1.7.0_45] at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) [rt.jar:1.7.0_45] at java.lang.reflect.Method.invoke(Unknown Source) [rt.jar:1.7.0_45] at org.jboss.as.ee.component.ManagedReferenceMethodInterceptorFactory$ManagedReferenceMethodInterceptor.processInvocation(ManagedReferenceMethodInterceptorFactory.java:72) [jboss-as-ee-7.1.0.Final.jar:7.1.0.Final] at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288) [jboss-invocation-1.1.1.Final.jar:1.1.1.Final] at org.jboss.invocation.WeavedInterceptor.processInvocation(WeavedInterceptor.java:53) [jboss-invocation-1.1.1.Final.jar:1.1.1.Final] at org.jboss.as.ee.component.interceptors.UserInterceptorFactory$1.processInvocation(UserInterceptorFactory.java:36) [jboss-as-ee-7.1.0.Final.jar:7.1.0.Final] at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288) [jboss-invocation-1.1.1.Final.jar:1.1.1.Final] at org.jboss.as.jpa.interceptor.SBInvocationInterceptor.processInvocation(SBInvocationInterceptor.java:47) [jboss-as-jpa-7.1.0.Final.jar:7.1.0.Final] at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288) [jboss-invocation-1.1.1.Final.jar:1.1.1.Final] at org.jboss.invocation.InitialInterceptor.processInvocation(InitialInterceptor.java:21) [jboss-invocation-1.1.1.Final.jar:1.1.1.Final] at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288) [jboss-invocation-1.1.1.Final.jar:1.1.1.Final] at org.jboss.invocation.ChainedInterceptor.processInvocation(ChainedInterceptor.java:61) [jboss-invocation-1.1.1.Final.jar:1.1.1.Final] at org.jboss.as.ee.component.interceptors.ComponentDispatcherInterceptor.processInvocation(ComponentDispatcherInterceptor.java:53) [jboss-as-ee-7.1.0.Final.jar:7.1.0.Final] at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288) [jboss-invocation-1.1.1.Final.jar:1.1.1.Final] at org.jboss.as.ejb3.component.pool.PooledInstanceInterceptor.processInvocation(PooledInstanceInterceptor.java:51) [jboss-as-ejb3-7.1.0.Final.jar:7.1.0.Final] at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288) [jboss-invocation-1.1.1.Final.jar:1.1.1.Final] at org.jboss.as.ejb3.tx.CMTTxInterceptor.invokeInOurTx(CMTTxInterceptor.java:228) [jboss-as-ejb3-7.1.0.Final.jar:7.1.0.Final] ... 35 more Caused by: org.hibernate.PersistentObjectException: detached entity passed to persist: metier.entities.Cours at org.hibernate.event.internal.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:141) [hibernate-core-4.0.1.Final.jar:4.0.1.Final] at org.hibernate.event.internal.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:78) [hibernate-core-4.0.1.Final.jar:4.0.1.Final] at org.hibernate.internal.SessionImpl.firePersist(SessionImpl.java:725) [hibernate-core-4.0.1.Final.jar:4.0.1.Final] at org.hibernate.internal.SessionImpl.persist(SessionImpl.java:701) [hibernate-core-4.0.1.Final.jar:4.0.1.Final] at org.hibernate.internal.SessionImpl.persist(SessionImpl.java:705) [hibernate-core-4.0.1.Final.jar:4.0.1.Final] at org.hibernate.ejb.AbstractEntityManagerImpl.persist(AbstractEntityManagerImpl.java:853) [hibernate-entitymanager-4.0.1.Final.jar:4.0.1.Final] ... 56 more