0

I am creating an application where I need to perform CRUD operations of an Entity Book. I am using spring data to perform CURD operations.This book entity refers to two external entities called Author and Publisher. Now when I hit save method on the Book entity is gives me following exception

org.hibernate.PersistentObjectException: detached entity passed to persist: com.shreyas.finder.domain.person.Author
at org.hibernate.event.internal.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:124) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
at org.hibernate.internal.SessionImpl.firePersist(SessionImpl.java:765) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
at org.hibernate.internal.SessionImpl.persist(SessionImpl.java:758) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
at org.hibernate.jpa.event.internal.core.JpaPersistEventListener$1.cascade(JpaPersistEventListener.java:80) ~[hibernate-entitymanager-5.0.12.Final.jar:5.0.12.Final]
at org.hibernate.engine.internal.Cascade.cascadeToOne(Cascade.java:398) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
at org.hibernate.engine.internal.Cascade.cascadeAssociation(Cascade.java:323) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
at org.hibernate.engine.internal.Cascade.cascadeProperty(Cascade.java:162) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
at org.hibernate.engine.internal.Cascade.cascade(Cascade.java:111) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
at org.hibernate.event.internal.AbstractSaveEventListener.cascadeBeforeSave(AbstractSaveEventListener.java:425) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
at org.hibernate.event.internal.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:249) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
at org.hibernate.event.internal.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:178) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
at org.hibernate.event.internal.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:109) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
at org.hibernate.jpa.event.internal.core.JpaPersistEventListener.saveWithGeneratedId(JpaPersistEventListener.java:67) ~[hibernate-entitymanager-5.0.12.Final.jar:5.0.12.Final]
at org.hibernate.event.internal.DefaultPersistEventListener.entityIsTransient(DefaultPersistEventListener.java:189) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
at org.hibernate.event.internal.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:132) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
at org.hibernate.event.internal.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:58) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
at org.hibernate.internal.SessionImpl.firePersist(SessionImpl.java:775) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
at org.hibernate.internal.SessionImpl.persist(SessionImpl.java:748) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
at org.hibernate.internal.SessionImpl.persist(SessionImpl.java:753) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
at org.hibernate.jpa.spi.AbstractEntityManagerImpl.persist(AbstractEntityManagerImpl.java:1146) ~[hibernate-entitymanager-5.0.12.Final.jar:5.0.12.Final]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_121]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_121]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_121]
at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_121]
at org.springframework.orm.jpa.ExtendedEntityManagerCreator$ExtendedEntityManagerInvocationHandler.invoke(ExtendedEntityManagerCreator.java:347) ~[spring-orm-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at com.sun.proxy.$Proxy78.persist(Unknown Source) ~[na:na]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_121]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_121]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_121]
at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_121]
at org.springframework.orm.jpa.SharedEntityManagerCreator$SharedEntityManagerInvocationHandler.invoke(SharedEntityManagerCreator.java:298) ~[spring-orm-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at com.sun.proxy.$Proxy78.persist(Unknown Source) ~[na:na]
at org.springframework.data.jpa.repository.support.SimpleJpaRepository.save(SimpleJpaRepository.java:508) ~[spring-data-jpa-1.11.3.RELEASE.jar:na]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_121]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_121]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_121]
at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_121]
at org.springframework.data.repository.core.support.RepositoryFactorySupport$QueryExecutorMethodInterceptor.executeMethodOn(RepositoryFactorySupport.java:504) ~[spring-data-commons-1.13.3.RELEASE.jar:na]
at org.springframework.data.repository.core.support.RepositoryFactorySupport$QueryExecutorMethodInterceptor.doInvoke(RepositoryFactorySupport.java:489) ~[spring-data-commons-1.13.3.RELEASE.jar:na]
at org.springframework.data.repository.core.support.RepositoryFactorySupport$QueryExecutorMethodInterceptor.invoke(RepositoryFactorySupport.java:461) ~[spring-data-commons-1.13.3.RELEASE.jar:na]
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179) ~[spring-aop-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.springframework.data.projection.DefaultMethodInvokingMethodInterceptor.invoke(DefaultMethodInvokingMethodInterceptor.java:61) ~[spring-data-commons-1.13.3.RELEASE.jar:na]
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179) ~[spring-aop-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.springframework.transaction.interceptor.TransactionInterceptor$1.proceedWithInvocation(TransactionInterceptor.java:99) ~[spring-tx-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:282) ~[spring-tx-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:96) ~[spring-tx-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179) ~[spring-aop-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.springframework.dao.support.PersistenceExceptionTranslationInterceptor.invoke(PersistenceExceptionTranslationInterceptor.java:136) ~[spring-tx-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179) ~[spring-aop-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.springframework.data.jpa.repository.support.CrudMethodMetadataPostProcessor$CrudMethodMetadataPopulatingMethodInterceptor.invoke(CrudMethodMetadataPostProcessor.java:133) ~[spring-data-jpa-1.11.3.RELEASE.jar:na]
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179) ~[spring-aop-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:92) ~[spring-aop-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179) ~[spring-aop-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.springframework.data.repository.core.support.SurroundingTransactionDetectorMethodInterceptor.invoke(SurroundingTransactionDetectorMethodInterceptor.java:57) ~[spring-data-commons-1.13.3.RELEASE.jar:na]
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179) ~[spring-aop-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:213) ~[spring-aop-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at com.sun.proxy.$Proxy85.save(Unknown Source) ~[na:na]
at com.shreyas.finder.service.book.BookServiceImpl.save(BookServiceImpl.java:103) ~[classes/:na]
at com.shreyas.finder.web.book.BookController.save(BookController.java:82) ~[classes/:na]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_121]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_121]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_121]
at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_121]
at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:205) ~[spring-web-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:133) ~[spring-web-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:97) ~[spring-webmvc-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:827) ~[spring-webmvc-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:738) ~[spring-webmvc-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85) ~[spring-webmvc-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:963) ~[spring-webmvc-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:897) ~[spring-webmvc-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970) ~[spring-webmvc-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:872) ~[spring-webmvc-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:661) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846) ~[spring-webmvc-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:742) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) ~[tomcat-embed-websocket-8.5.14.jar:8.5.14]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99) ~[spring-web-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
at org.springframework.web.filter.HttpPutFormContentFilter.doFilterInternal(HttpPutFormContentFilter.java:105) ~[spring-web-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:81) ~[spring-web-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:197) ~[spring-web-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:198) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) [tomcat-embed-core-8.5.14.jar:8.5.14]
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:478) [tomcat-embed-core-8.5.14.jar:8.5.14]
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140) [tomcat-embed-core-8.5.14.jar:8.5.14]
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:80) [tomcat-embed-core-8.5.14.jar:8.5.14]
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87) [tomcat-embed-core-8.5.14.jar:8.5.14]
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342) [tomcat-embed-core-8.5.14.jar:8.5.14]
at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:799) [tomcat-embed-core-8.5.14.jar:8.5.14]
at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) [tomcat-embed-core-8.5.14.jar:8.5.14]
at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:861) [tomcat-embed-core-8.5.14.jar:8.5.14]
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1455) [tomcat-embed-core-8.5.14.jar:8.5.14]
at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-8.5.14.jar:8.5.14]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [na:1.8.0_121]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [na:1.8.0_121]
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-8.5.14.jar:8.5.14]
at java.lang.Thread.run(Thread.java:745) [na:1.8.0_121]

Here is my code :

Book.java

@Entity
@Table(name = "book")
public class Book {

  @Id
  @GeneratedValue(strategy = GenerationType.AUTO)
  private long id;

  @Column(name = "book_title", nullable = false)
  private String bookTitle;

  @Column(name = "book_description", nullable = true)
  private String bookDescription;

  @Column(name = "book_unit_price", nullable = false)
  private Float bookUnitPrice;

  @OneToOne(cascade = CascadeType.ALL)
  @JoinColumn(name = "author_id")
  private Author author;

  @OneToOne(cascade = CascadeType.ALL)
  @JoinColumn(name = "publisher_id")
  private Publisher publisher;
}

BookDAO.java

@Repository("bookDao")
public interface BookDAO extends JpaRepository<Book, Long> {

  public List<Book> findByBookTitle(String name);

  public List<Book> findByBookUnitPrice(Float price);

  public List<Book> findByBookUnitPriceBetween(Float start , Float end);
}

BookServiceImpl.java has the following implementation of save method

@Override
public Book save(Book book) {
  if(!bookDao.exists(book.getId()))
    return bookDao.save(book);
  else {
    Book existingBook = bookDao.findOne(book.getId());
    existingBook.setBookTitle(book.getBookTitle());
    existingBook.setBookDescription(book.getBookDescription());
    existingBook.setAuthor(book.getAuthor());
    existingBook.setPublisher(book.getPublisher());
    return bookDao.save(existingBook);
  }
}

BookController.java has the following implementation of save rest endpoint.

@RequestMapping(value = "/save", method = RequestMethod.POST, consumes = "application/json",
  produces = "application/json")
public ResponseEntity<Book> save(@RequestBody Book book) {
  return new ResponseEntity<Book>(bookService.save(book), HttpStatus.OK);
}

And finnaly the payload I am passing to the save method.

{
  "bookTitle": "Java Complete Reference",
  "bookDescription": "A great book to start java for beginners",
  "bookUnitPrice": 100,
  "author": {
    "id": 1,
    "firstName": "First Name",
    "lastName": "Last Name",
    "bio": "Author Bio"
  },
  "publisher": {
    "id": 4,
    "publisherName": "Continental Publications"
  }
}

When I pass this payload via HTTP post method to the rest endpoint I get the exception mentioned above. Now I want to persist a Book entity which refers to the existing Author and Publisher entity. I have searched for some problem of similar kind but none of the solutions are of my help.Can someone please suggest what am I doing wrong and what needs to be changed?

Thanks for the help in advance.

Shreyas Nargund
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  • another thing I would like to mention is that other operations for this entity are working fine the problem is with the create and update operations.I am getting correct results for get methods – Shreyas Nargund Jun 09 '17 at 09:06
  • Please check here... https://stackoverflow.com/q/13370221/7504001 – Vikas Suryawanshi Jun 09 '17 at 09:08
  • @VikasSuryawanshi thanks for the quick reply.I just wanted to make sure of I am passing the payload in correct format? – Shreyas Nargund Jun 09 '17 at 09:10
  • Your payload does not contain the book `id` information. Since your `id` field inside `Book` is a primitive long, `if(!bookDao.exists(book.getId()))` will always equals `if(!bookDao.exists(0L))` since the `id` field is missing. Your test should be `if(book.getId() != null && !bookDao.exists(book.getId()))` and your payload should contains an `id` field. – kagmole Jun 09 '17 at 09:20

2 Answers2

0

I was going through this question on stackoverflow and tried one of its solution and it worked for me. Replacing

@OneToOne(cascade = CascadeType.ALL)

with

@OneToOne(cascade = CascadeType.MERGE)

worked for me. Now I don't know if it is a Hot-fix or why I didn't work with CascadeType.ALL

Shreyas Nargund
  • 145
  • 16
  • You're passing `book.getAuthor()` and `book.getPublisher()` both detached in case of update, so that's why you're getting exception. You should also retrieve them from database and just update fields. – Orest Jun 09 '17 at 09:19
0

This is a general architecural problem which I see rarely addressed.

The point is that your JPA entities can be in several states:

  1. transient only
  2. persistent unchanged attached
  3. persistent unchanged detached
  4. persistent changed detached
  5. persistent changed attached unflushed
  6. persistent changed attached flushed

Often I see JPA entities passed around and even serialized so they exit the JPA context an become "detached".

You should have a clear answer to ANY state of your JPA Entity in the architectural context. If you do not adress it it becomes micro management and your code will be more and more inexpressive as you always have to be aware which method is save to call in which state of the JPA entity.

To your question:

A simple possibility would be to load the entity by id to get an attached representation and map the values from the detached entity to the attached entity.

The real problem is that your entity quietly became a communication object ( I call it suggestion or proposal object). It is an object that works together in the context of the operation to perform (create/update/delete/read). That said it is only tip of the architectural iceberg.

oopexpert
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