title: Persistance JDO - How to query a property of a collection with JDOQL?
body: <p>I want to build an application where a user identified by an email address can have several application accounts. Each account can have one or more users. I am trying to use the JDO Storage capabilities with Google App Engine Java. Here is my attempt:</p>
  
  <pre><code>@PersistenceCapable
  @Inheritance(strategy = InheritanceStrategy.NEW_TABLE)
  public class AppAccount {
       @PrimaryKey
       @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
       private Long id;
  
      @Persistent
      private String companyName;
  
      @Persistent
      List&lt;Invoices&gt; invoices = new ArrayList&lt;Invoices&gt;();
  
      @Persistent
      List&lt;AppUser&gt; users = new ArrayList&lt;AppUser&gt;();
  
      // Getter Setters and Other Fields
  }
  
  @PersistenceCapable
  @EmbeddedOnly
  public class AppUser {
  
      @Persistent
      private String username;
  
      @Persistent
      private String firstName;
  
      @Persistent
      private String lastName;
  
       // Getter Setters and Other Fields
  }
  </code></pre>
  
  <p>When a user logs in, I want to check how many accounts he belongs to. If he or she belongs to more than one then he or she will be presented with a dashboard where he/she can click which account he/she wants to load. This is my code to retrieve a list of app accounts where he/she is registered.</p>
  
  <pre><code>public static List&lt;AppAccount&gt; getUserAppAccounts(String username) {
      PersistenceManager pm = JdoUtil.getPm();
      Query q = pm.newQuery(AppAccount.class);
      q.setFilter("users.username == usernameParam");
      q.declareParameters("String usernameParam");
      return (List&lt;AppAccount&gt;) q.execute(username);
  }
  </code></pre>
  
  <p>But I get the next error: </p>
  
  <pre><code>SELECT FROM invoices.server.AppAccount WHERE users.username == usernameParam PARAMETERS String usernameParam: Encountered a variable expression that isn't part of a join.  Maybe you're referencing a non-existent field of an embedded class.
  org.datanucleus.store.appengine.FatalNucleusUserException: SELECT FROM com.softamo.pelicamo.invoices.server.AppAccount WHERE users.username == usernameParam PARAMETERS String usernameParam: Encountered a variable expression that isn't part of a join.  Maybe you're referencing a non-existent field of an embedded class.
      at org.datanucleus.store.appengine.query.DatastoreQuery.getJoinClassMetaData(DatastoreQuery.java:1154)
      at org.datanucleus.store.appengine.query.DatastoreQuery.addLeftPrimaryExpression(DatastoreQuery.java:1066)
      at org.datanucleus.store.appengine.query.DatastoreQuery.addExpression(DatastoreQuery.java:846)
      at org.datanucleus.store.appengine.query.DatastoreQuery.addFilters(DatastoreQuery.java:807)
      at org.datanucleus.store.appengine.query.DatastoreQuery.performExecute(DatastoreQuery.java:226)
      at org.datanucleus.store.appengine.query.JDOQLQuery.performExecute(JDOQLQuery.java:85)
      at org.datanucleus.store.query.Query.executeQuery(Query.java:1489)
      at org.datanucleus.store.query.Query.executeWithArray(Query.java:1371)
      at org.datanucleus.jdo.JDOQuery.execute(JDOQuery.java:243)
      at com.softamo.pelicamo.invoices.server.Store.getUserAppAccounts(Store.java:82)
      at com.softamo.pelicamo.invoices.test.server.StoreTest.testgetUserAppAccounts(StoreTest.java:39)
      at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
      at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
      at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
      at java.lang.reflect.Method.invoke(Method.java:597)
      at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:44)
      at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
      at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:41)
      at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
      at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28)
      at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:31)
      at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:76)
      at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
      at org.junit.runners.ParentRunner$3.run(ParentRunner.java:193)
      at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:52)
      at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:191)
      at org.junit.runners.ParentRunner.access$000(ParentRunner.java:42)
      at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:184)
      at org.junit.runners.ParentRunner.run(ParentRunner.java:236)
      at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:46)
      at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
      at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
      at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
      at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
      at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)
  </code></pre>
  
  <p>Any ideas?</p>
  
  <p>Am I getting JDO persistance totally wrong?</p>