title: Exception : "Missing class indicator field from database row [UnmarshalRecordImpl()]." when unmarshalling XML using EclipseLink JAXB (MOXy)
body: <p>Is it any way to unmarshalling with @XmlDescriminatorNode/@XmlDescrimintatorValue annotations next XML, or any workaround:</p>
  
  <pre class="lang-xml prettyprint-override"><code>&lt;assets&gt;
      &lt;asset type="full"&gt;
          &lt;data_file role="source"&gt;
              &lt;locale name="ru-RU"/&gt;
          &lt;/data_file&gt;
          &lt;data_file role="extension"&gt;
              &lt;locale name="ru-RU"/&gt;
          &lt;/data_file&gt;
          &lt;data_file&gt;
              &lt;locale name="ru-RU"/&gt;
          &lt;/data_file&gt;
      &lt;/asset&gt;
  &lt;/assets&gt;
  </code></pre>
  
  <p>My mapping classes:
  </p>
  
  <pre><code>@XmlRootElement(name="data_file")
  @XmlAccessorType(XmlAccessType.FIELD)
  @XmlDiscriminatorNode("@role")
  public abstract class BaseDataFile implements Serializable {
  
      @XmlPath("@role")
      @XmlAttribute(name = "role")
      private String role;
  
      public String getRole() {
          return role;
      }
  
      public void setRole(String role) {
          this.role = role;
      }
  }
  </code></pre>
  
  <pre class="lang-java prettyprint-override"><code>@XmlRootElement(name="data_file")
  @XmlAccessorType(XmlAccessType.FIELD)
  @XmlDiscriminatorValue("source")
  public class SourceDataFile extends BaseDataFile {
  
  }
  </code></pre>
  
  <pre class="lang-java prettyprint-override"><code>@XmlRootElement(name="data_file")
  @XmlAccessorType(XmlAccessType.FIELD)
  @XmlDiscriminatorValue("source")
  public class SourceDataFile extends BaseDataFile {
  
  }
  </code></pre>
  
  <pre class="lang-java prettyprint-override"><code>@XmlRootElement(name="data_file")
  @XmlAccessorType(XmlAccessType.FIELD)
  @XmlDiscriminatorValue("extension")
  public class SourceDataFile extends BaseDataFile {
  
  }
  </code></pre>
  
  <pre class="lang-java prettyprint-override"><code>@XmlRootElement(name="asset")
  @XmlAccessorType(XmlAccessType.FIELD)
  @XmlDiscriminatorNode("@type")
  public abstract class BaseAsset implements Serializable {
  
      @XmlPath("@type")
      @XmlAttribute(name = "type")
      private String type;
  
      @XmlPath("data_file")
      private List&lt;BaseDataFile&gt; dataFiles;
  
      public List&lt;BaseDataFile&gt; getDataFiles() {
          return dataFiles;
      }
  
      public void setDataFiles(List&lt;BaseDataFile&gt; dataFiles) {
          this.dataFiles = dataFiles;
      }
  
      public String getType() {
          return type;
      }
  
      public void setType(String type) {
          this.type = type;
      }
  }
  </code></pre>
  
  <p>The error occures if XML containes element as below, without <strong>"type"</strong> attribute:
  </p>
  
  <pre><code>&lt;data_file&gt;
    &lt;locale name="ru-RU"/&gt;
  &lt;/data_file&gt;
  </code></pre>
  
  <p>Thanks in advance</p>