JPQL query return null when get list of entity

[Klong Vu]

My entity class :

public class ACCOUNT implements Serializable{

private static final long serialVersionUID = 1L;

@Id

@Column(name = "USERNAME")

private String Username;

@Column(name = "PASSWORD")

private String Password;

public ACCOUNT(String user,String pass)

{

this.Username=user;

this.Password=pass;

}

    // geter and setter

my persistence.xml

<?xml version="1.0" encoding="UTF-8" ?>

<persistence xmlns="http://java.sun.com/xml/ns/persistence"

    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

    xsi:schemaLocation="http://java.sun.com/xml/ns/persistence

        http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd" version="1.0">

    <persistence-unit name="transactions-optional">

        <provider>org.datanucleus.api.jpa.PersistenceProviderImpl</provider>

        <class>com.materialshop.server.Datastore.ACCOUNT</class>

        <exclude-unlisted-classes/>

        <properties>

            <property name="datanucleus.NontransactionalRead" value="true"/>

            <property name="datanucleus.NontransactionalWrite" value="true"/>

            <property name="datanucleus.nontx.atomic" value="true"/>

            <property name="datanucleus.ConnectionURL" value="appengine"/>

        </properties>

    </persistence-unit>

</persistence>

Now persist account and it was successful

ACCOUNT ac=new ACCOUNT("admin","123");

em.persist(ac);

I checked http://localhost:8888/_ah/admin and yes ,there is ACOUNT entity with 1 record admin and 123

but when I use JPQL to get all entity , it returned null

Query q=new Query("SELECT ac FROM ACCOUNT ac");

List<ACCOUNT> list=q.getResultList();

even with 

em.find(ACCOUNT.class,"admin"); return null too 

Did i miss something ? Please help me , very grateful for your help