attempted to assign id from null one-to-one property [com.coderscampus.domain.Cart.user]
<p>I'm building out a simple eCommerce web app for fun, but for some reason I'm getting an exception with my @OneToOne relationship between a <code>User</code> and a shopping <code>Cart</code></p>

<p>I'm pretty well versed with Hibernate and bi-directional relationships, so I do know that you need to set each relationship in the bi-directional relationship... but it still seems to fail.</p>

<p>Here's my User / Cart objects and the Controller code in question:</p>

<p><strong>User.class</strong></p>

<pre><code>import java.util.HashSet;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import javax.persistence.OneToOne;

import com.coderscampus.security.Authority;
import com.fasterxml.jackson.annotation.JsonIdentityInfo;
import com.fasterxml.jackson.annotation.ObjectIdGenerators;

@Entity
@JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class, property = "id")
public class User
{
  private Long id;
  private String password;
  private String email;
  private Set&lt;Product&gt; products = new HashSet&lt;&gt;();
  private Set&lt;Review&gt; reviews = new HashSet&lt;&gt;();
  private Cart cart;
  // OneToMany relationship between a User and its Authorities
  private Set&lt;Authority&gt; authorities = new HashSet&lt;&gt;();
  private Set&lt;Order&gt; orders = new HashSet&lt;&gt;();

  @Id
  @GeneratedValue
  public Long getId()
  {
    return id;
  }

  public void setId(Long id)
  {
    this.id = id;
  }

  public String getPassword()
  {
    return password;
  }

  public void setPassword(String password)
  {
    this.password = password;
  }

  public String getEmail()
  {
    return email;
  }

  public void setEmail(String email)
  {
    this.email = email;
  }

  @OneToMany(cascade=CascadeType.ALL, mappedBy="user", fetch=FetchType.EAGER)
  public Set&lt;Product&gt; getProducts()
  {
    return products;
  }

  public void setProducts(Set&lt;Product&gt; products)
  {
    this.products = products;
  }

  @OneToMany(cascade=CascadeType.ALL, mappedBy="user", fetch=FetchType.EAGER)
  public Set&lt;Review&gt; getReviews()
  {
    return reviews;
  }

  public void setReviews(Set&lt;Review&gt; reviews)
  {
    this.reviews = reviews;
  }

  @OneToOne(cascade=CascadeType.ALL, mappedBy="user", fetch=FetchType.EAGER)
  public Cart getCart()
  {
    return cart;
  }

  public void setCart(Cart cart)
  {
    this.cart = cart;
  }

  @OneToMany(cascade=CascadeType.ALL, fetch=FetchType.EAGER, mappedBy="user")
  public Set&lt;Authority&gt; getAuthorities()
  {
    return authorities;
  }

  public void setAuthorities(Set&lt;Authority&gt; authorities)
  {
    this.authorities = authorities;
  }

  @OneToMany(cascade=CascadeType.ALL, fetch=FetchType.EAGER, mappedBy="user")
  public Set&lt;Order&gt; getOrders()
  {
    return orders;
  }

  public void setOrders(Set&lt;Order&gt; orders)
  {
    this.orders = orders;
  }
}
</code></pre>

<p><strong>Cart.class</strong></p>

<pre><code>import java.util.Date;
import java.util.HashSet;
import java.util.Set;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.OneToOne;
import javax.persistence.PrimaryKeyJoinColumn;

import org.hibernate.annotations.GenericGenerator;
import org.hibernate.annotations.Parameter;

import com.fasterxml.jackson.annotation.JsonIdentityInfo;
import com.fasterxml.jackson.annotation.ObjectIdGenerators;

@Entity
@JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class, property = "id")
public class Cart
{
  private Long id;
  private Integer quantity;
  private Date dateAdded;
  private User user;
  private Set&lt;Product&gt; products = new HashSet&lt;&gt;();

  @Id  
  @Column(name="user_id")
  @GeneratedValue(generator="myGenerator")  
  @GenericGenerator(name="myGenerator", strategy="foreign", parameters=@Parameter(value="user", name = "property"))
  public Long getId()
  {
    return id;
  }

  public void setId(Long id)
  {
    this.id = id;
  }

  public Integer getQuantity()
  {
    return quantity;
  }

  public void setQuantity(Integer quantity)
  {
    this.quantity = quantity;
  }

  public Date getDateAdded()
  {
    return dateAdded;
  }

  public void setDateAdded(Date dateAdded)
  {
    this.dateAdded = dateAdded;
  }

  @OneToOne
  @PrimaryKeyJoinColumn
  public User getUser()
  {
    return user;
  }

  public void setUser(User user)
  {
    this.user = user;
  }

  @ManyToMany
  @JoinTable(name="product_cart", joinColumns=@JoinColumn(name="cart_id"), inverseJoinColumns=@JoinColumn(name="product_id"))
  public Set&lt;Product&gt; getProducts()
  {
    return products;
  }

  public void setProducts(Set&lt;Product&gt; products)
  {
    this.products = products;
  }
}
</code></pre>

<p><strong>Controller method</strong></p>

<pre><code>@RequestMapping(value="{productId}", method=RequestMethod.POST)
  public @ResponseBody Cart addProductToCart (@AuthenticationPrincipal User user, 
      HttpServletRequest request, @PathVariable Long productId, ModelMap model)
  {
    Cart cart = new Cart();
    // if user isn't logged in, then have them login or create an account
    if (user == null)
    {
      return null;
    }
    else
    {
      // store cart information on user domain object
      user = userRepo.findOne(user.getId());

      // create the shopping cart object and populate it with the product
      user.setCart(cart);
      cart.setUser(user);

      user = userRepo.save(user); //exception happens on save here
    }

    return cart;
  }
</code></pre>

<p>Here's the exception:</p>

<pre><code>org.hibernate.id.IdentifierGenerationException: attempted to assign id from null one-to-one property [com.coderscampus.domain.Cart.user]
    at org.hibernate.id.ForeignGenerator.generate(ForeignGenerator.java:98) ~[hibernate-core-4.3.11.Final.jar:4.3.11.Final]
    at org.hibernate.event.internal.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:118) ~[hibernate-core-4.3.11.Final.jar:4.3.11.Final]
    at org.hibernate.jpa.event.internal.core.JpaMergeEventListener.saveWithGeneratedId(JpaMergeEventListener.java:73) ~[hibernate-entitymanager-4.3.11.Final.jar:4.3.11.Final]
    at org.hibernate.event.internal.DefaultMergeEventListener.saveTransientEntity(DefaultMergeEventListener.java:271) ~[hibernate-core-4.3.11.Final.jar:4.3.11.Final]
    at org.hibernate.event.internal.DefaultMergeEventListener.entityIsTransient(DefaultMergeEventListener.java:251) ~[hibernate-core-4.3.11.Final.jar:4.3.11.Final]
    at org.hibernate.event.internal.DefaultMergeEventListener.onMerge(DefaultMergeEventListener.java:189) ~[hibernate-core-4.3.11.Final.jar:4.3.11.Final]
    at org.hibernate.internal.SessionImpl.fireMerge(SessionImpl.java:886) ~[hibernate-core-4.3.11.Final.jar:4.3.11.Final]
    at org.hibernate.internal.SessionImpl.merge(SessionImpl.java:868) ~[hibernate-core-4.3.11.Final.jar:4.3.11.Final]
    at org.hibernate.engine.spi.CascadingActions$6.cascade(CascadingActions.java:277) ~[hibernate-core-4.3.11.Final.jar:4.3.11.Final]
    at org.hibernate.engine.internal.Cascade.cascadeToOne(Cascade.java:350) ~[hibernate-core-4.3.11.Final.jar:4.3.11.Final]
    at org.hibernate.engine.internal.Cascade.cascadeAssociation(Cascade.java:293) ~[hibernate-core-4.3.11.Final.jar:4.3.11.Final]
    at org.hibernate.engine.internal.Cascade.cascadeProperty(Cascade.java:161) ~[hibernate-core-4.3.11.Final.jar:4.3.11.Final]
    at org.hibernate.engine.internal.Cascade.cascade(Cascade.java:118) ~[hibernate-core-4.3.11.Final.jar:4.3.11.Final]
    at org.hibernate.event.internal.DefaultMergeEventListener.cascadeOnMerge(DefaultMergeEventListener.java:474) ~[hibernate-core-4.3.11.Final.jar:4.3.11.Final]
    at org.hibernate.event.internal.DefaultMergeEventListener.entityIsPersistent(DefaultMergeEventListener.java:218) ~[hibernate-core-4.3.11.Final.jar:4.3.11.Final]
    at org.hibernate.event.internal.DefaultMergeEventListener.onMerge(DefaultMergeEventListener.java:192) ~[hibernate-core-4.3.11.Final.jar:4.3.11.Final]
    at org.hibernate.event.internal.DefaultMergeEventListener.onMerge(DefaultMergeEventListener.java:85) ~[hibernate-core-4.3.11.Final.jar:4.3.11.Final]
    at org.hibernate.internal.SessionImpl.fireMerge(SessionImpl.java:876) ~[hibernate-core-4.3.11.Final.jar:4.3.11.Final]
    at org.hibernate.internal.SessionImpl.merge(SessionImpl.java:858) ~[hibernate-core-4.3.11.Final.jar:4.3.11.Final]
    at org.hibernate.internal.SessionImpl.merge(SessionImpl.java:863) ~[hibernate-core-4.3.11.Final.jar:4.3.11.Final]
    at org.hibernate.jpa.spi.AbstractEntityManagerImpl.merge(AbstractEntityManagerImpl.java:1196) ~[hibernate-entitymanager-4.3.11.Final.jar:4.3.11.Final]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_91]
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[na:1.8.0_91]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[na:1.8.0_91]
    at java.lang.reflect.Method.invoke(Unknown Source) ~[na:1.8.0_91]
    at org.springframework.orm.jpa.ExtendedEntityManagerCreator$ExtendedEntityManagerInvocationHandler.invoke(ExtendedEntityManagerCreator.java:344) ~[spring-orm-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at com.sun.proxy.$Proxy84.merge(Unknown Source) ~[na:na]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_91]
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[na:1.8.0_91]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[na:1.8.0_91]
    at java.lang.reflect.Method.invoke(Unknown Source) ~[na:1.8.0_91]
    at org.springframework.orm.jpa.SharedEntityManagerCreator$SharedEntityManagerInvocationHandler.invoke(SharedEntityManagerCreator.java:294) ~[spring-orm-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at com.sun.proxy.$Proxy84.merge(Unknown Source) ~[na:na]
    at org.springframework.data.jpa.repository.support.SimpleJpaRepository.save(SimpleJpaRepository.java:442) ~[spring-data-jpa-1.9.4.RELEASE.jar:na]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_91]
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[na:1.8.0_91]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[na:1.8.0_91]
    at java.lang.reflect.Method.invoke(Unknown Source) ~[na:1.8.0_91]
    at org.springframework.data.repository.core.support.RepositoryFactorySupport$QueryExecutorMethodInterceptor.executeMethodOn(RepositoryFactorySupport.java:483) ~[spring-data-commons-1.11.4.RELEASE.jar:na]
    at org.springframework.data.repository.core.support.RepositoryFactorySupport$QueryExecutorMethodInterceptor.doInvoke(RepositoryFactorySupport.java:468) ~[spring-data-commons-1.11.4.RELEASE.jar:na]
    at org.springframework.data.repository.core.support.RepositoryFactorySupport$QueryExecutorMethodInterceptor.invoke(RepositoryFactorySupport.java:440) ~[spring-data-commons-1.11.4.RELEASE.jar:na]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179) ~[spring-aop-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.springframework.data.projection.DefaultMethodInvokingMethodInterceptor.invoke(DefaultMethodInvokingMethodInterceptor.java:61) ~[spring-data-commons-1.11.4.RELEASE.jar:na]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179) ~[spring-aop-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.springframework.transaction.interceptor.TransactionInterceptor$1.proceedWithInvocation(TransactionInterceptor.java:99) ~[spring-tx-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:281) ~[spring-tx-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:96) ~[spring-tx-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179) ~[spring-aop-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.springframework.dao.support.PersistenceExceptionTranslationInterceptor.invoke(PersistenceExceptionTranslationInterceptor.java:136) ~[spring-tx-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179) ~[spring-aop-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.springframework.data.jpa.repository.support.CrudMethodMetadataPostProcessor$CrudMethodMetadataPopulatingMethodInterceptor.invoke(CrudMethodMetadataPostProcessor.java:131) ~[spring-data-jpa-1.9.4.RELEASE.jar:na]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179) ~[spring-aop-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:92) ~[spring-aop-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179) ~[spring-aop-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:208) ~[spring-aop-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at com.sun.proxy.$Proxy87.save(Unknown Source) ~[na:na]
    at com.coderscampus.web.ConsumerProductController.addProductToCart(ConsumerProductController.java:60) ~[classes/:na]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_91]
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[na:1.8.0_91]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[na:1.8.0_91]
    at java.lang.reflect.Method.invoke(Unknown Source) ~[na:1.8.0_91]
    at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:221) ~[spring-web-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:136) ~[spring-web-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:110) ~[spring-webmvc-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:832) ~[spring-webmvc-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:743) ~[spring-webmvc-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85) ~[spring-webmvc-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:961) ~[spring-webmvc-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:895) ~[spring-webmvc-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:967) ~[spring-webmvc-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:869) ~[spring-webmvc-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:648) ~[tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:843) ~[spring-webmvc-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:729) ~[tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:292) ~[tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207) ~[tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) ~[tomcat-embed-websocket-8.0.33.jar:8.0.33]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240) ~[tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207) ~[tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:316) ~[spring-security-web-4.0.4.RELEASE.jar:4.0.4.RELEASE]
    at org.springframework.security.web.access.intercept.FilterSecurityInterceptor.invoke(FilterSecurityInterceptor.java:126) ~[spring-security-web-4.0.4.RELEASE.jar:4.0.4.RELEASE]
    at org.springframework.security.web.access.intercept.FilterSecurityInterceptor.doFilter(FilterSecurityInterceptor.java:90) ~[spring-security-web-4.0.4.RELEASE.jar:4.0.4.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330) ~[spring-security-web-4.0.4.RELEASE.jar:4.0.4.RELEASE]
    at org.springframework.security.web.access.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:114) ~[spring-security-web-4.0.4.RELEASE.jar:4.0.4.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330) ~[spring-security-web-4.0.4.RELEASE.jar:4.0.4.RELEASE]
    at org.springframework.security.web.session.SessionManagementFilter.doFilter(SessionManagementFilter.java:122) ~[spring-security-web-4.0.4.RELEASE.jar:4.0.4.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330) ~[spring-security-web-4.0.4.RELEASE.jar:4.0.4.RELEASE]
    at org.springframework.security.web.authentication.AnonymousAuthenticationFilter.doFilter(AnonymousAuthenticationFilter.java:111) ~[spring-security-web-4.0.4.RELEASE.jar:4.0.4.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330) ~[spring-security-web-4.0.4.RELEASE.jar:4.0.4.RELEASE]
    at org.springframework.security.web.servletapi.SecurityContextHolderAwareRequestFilter.doFilter(SecurityContextHolderAwareRequestFilter.java:169) ~[spring-security-web-4.0.4.RELEASE.jar:4.0.4.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330) ~[spring-security-web-4.0.4.RELEASE.jar:4.0.4.RELEASE]
    at org.springframework.security.web.savedrequest.RequestCacheAwareFilter.doFilter(RequestCacheAwareFilter.java:48) ~[spring-security-web-4.0.4.RELEASE.jar:4.0.4.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330) ~[spring-security-web-4.0.4.RELEASE.jar:4.0.4.RELEASE]
    at org.springframework.security.web.authentication.AbstractAuthenticationProcessingFilter.doFilter(AbstractAuthenticationProcessingFilter.java:205) ~[spring-security-web-4.0.4.RELEASE.jar:4.0.4.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330) ~[spring-security-web-4.0.4.RELEASE.jar:4.0.4.RELEASE]
    at org.springframework.security.web.authentication.logout.LogoutFilter.doFilter(LogoutFilter.java:120) ~[spring-security-web-4.0.4.RELEASE.jar:4.0.4.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330) ~[spring-security-web-4.0.4.RELEASE.jar:4.0.4.RELEASE]
    at org.springframework.security.web.header.HeaderWriterFilter.doFilterInternal(HeaderWriterFilter.java:64) ~[spring-security-web-4.0.4.RELEASE.jar:4.0.4.RELEASE]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330) ~[spring-security-web-4.0.4.RELEASE.jar:4.0.4.RELEASE]
    at org.springframework.security.web.context.SecurityContextPersistenceFilter.doFilter(SecurityContextPersistenceFilter.java:91) ~[spring-security-web-4.0.4.RELEASE.jar:4.0.4.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330) ~[spring-security-web-4.0.4.RELEASE.jar:4.0.4.RELEASE]
    at org.springframework.security.web.context.request.async.WebAsyncManagerIntegrationFilter.doFilterInternal(WebAsyncManagerIntegrationFilter.java:53) ~[spring-security-web-4.0.4.RELEASE.jar:4.0.4.RELEASE]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330) ~[spring-security-web-4.0.4.RELEASE.jar:4.0.4.RELEASE]
    at org.springframework.security.web.FilterChainProxy.doFilterInternal(FilterChainProxy.java:213) ~[spring-security-web-4.0.4.RELEASE.jar:4.0.4.RELEASE]
    at org.springframework.security.web.FilterChainProxy.doFilter(FilterChainProxy.java:176) ~[spring-security-web-4.0.4.RELEASE.jar:4.0.4.RELEASE]
    at org.springframework.web.filter.DelegatingFilterProxy.invokeDelegate(DelegatingFilterProxy.java:346) ~[spring-web-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.springframework.web.filter.DelegatingFilterProxy.doFilter(DelegatingFilterProxy.java:262) ~[spring-web-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240) ~[tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207) ~[tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99) ~[spring-web-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240) ~[tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207) ~[tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.springframework.web.filter.HttpPutFormContentFilter.doFilterInternal(HttpPutFormContentFilter.java:87) ~[spring-web-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240) ~[tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207) ~[tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:77) ~[spring-web-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240) ~[tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207) ~[tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:121) ~[spring-web-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-4.2.6.RELEASE.jar:4.2.6.RELEASE]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240) ~[tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207) ~[tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:212) ~[tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106) [tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502) [tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:141) [tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79) [tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88) [tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:522) [tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1095) [tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:672) [tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1502) [tomcat-embed-core-8.0.33.jar:8.0.33]
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1458) [tomcat-embed-core-8.0.33.jar:8.0.33]
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source) [na:1.8.0_91]
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source) [na:1.8.0_91]
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-8.0.33.jar:8.0.33]
    at java.lang.Thread.run(Unknown Source) [na:1.8.0_91]
</code></pre>

<p>I can confirm that the user is loaded properly from the DB and the <code>Cart</code> is a brand new object.</p>

<p>So the <code>Cart</code> gets a <code>User</code> set into it, and the <code>User</code> gets that <code>Cart</code> set into it... I've also tried adding <code>cart.setId(user.getId())</code> before the save, but that doesn't help either.</p>