If you are creating JPA entities, make sure to add the class to the mapping file of the persistence unit for its data source.

Example configurations on SDK Embedded Server persistence.xml

8<---
    <persistence-unit name="user-unit" transaction-type="JTA">
        <provider>org.hibernate.ejb.HibernatePersistence</provider>
        <jta-data-source>jdbc/USERS_DS</jta-data-source>
        <mapping-file>META-INF/user-entities.xml</mapping-file>
        <validation-mode>NONE</validation-mode>
---->8

user-entities.xml

8<---
<?xml version="1.0" encoding="UTF-8" ?>
<entity-mappings xmlns="http://java.sun.com/xml/ns/persistence/orm"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://java.sun.com/xml/ns/persistence/orm http://java.sun.com/xml/ns/persistence/orm_2_0.xsd"
  version="2.0">
    <entity class="org.eko.orderusersapp.entity.User" />
</entity-mappings>
---->8

Entity

@Entity
@Table(name = "ORDERS_USER")
public class User {
 
  @Id
  private Integer id;
  @Column(name = "FIRST_NAME")
  private String firstName;
 
  public Integer getId() {
    return id;
  }
  public String getFirstName() {
    return firstName;
  }
}