title: Activiti - HistoricTaskInstanceQuery, search by property of variable which is JPA entity
body: <p>I have a task with variable which is JPA entity.</p>
  
  <p>What the HistoricTaskInstanceQuery API provides is only search by the entity itself:</p>
  
  <pre><code>query.processVariableValueEquals("entity", entity);
  </code></pre>
  
  <p>My goal is to search by the entity attributes, something like (pseudo code):</p>
  
  <pre><code>query.processVariableAttributeEquals("entity", "entity.subEntity.name", name)
  </code></pre>
  
  <p>How can I achieve this ?</p>