title: Issue with a @ManyToMany relationship persisting into reference tables
body: <p>I have a <strong>Curriculum entity</strong> which is as follows:</p>
  
  <pre><code>@Entity
  public class Curriculum {
  
      @ManyToMany
      private Set&lt;Language&gt; languages;
      ...
  </code></pre>
  
  <p>I am trying to persist a Curriculum instance but <strong>the constraint is that the Language instances are already in database</strong>.</p>
  
  <p>How can I make sure that when a call to persist is made, a line is inserted into the <code>curriculum</code> table, the <code>curriculum_languages</code> mapping table <strong>but not into</strong> the <code>language</code> table as it is a reference table which is already populated?</p>
  
  <p><strong>edit 1</strong>: </p>
  
  <p>-Here is the error I get if I don't specify the <code>Cascade.ALL</code> attribute:</p>
  
  <pre><code>org.hibernate.TransientObjectException: object references an unsaved transient instance - save the transient instance before flushing: com.bignibou.domain.Language
  </code></pre>
  
  <p>-If I do specify the <code>Cascade.ALL</code>, new lines with new IDs are insterted into the <code>language</code> table which is obviously not what I want...</p>
  
  <p><strong>edit 2</strong>: Note that I use Spring Data JPA in order to persist my instances and the data coming from the browser is a JSON object as follows:</p>
  
  <pre><code>{"curriculum":{"languages":[{"id":46,"description":"Français"},{"id":30,"description":"Chinois"}],"firstName":"Julianito","dateOfBirth":"1975-01-06","telephoneNumber":"0608965874","workExperienceInYears":3,"maxNumberChildren":1,"drivingLicense":true}}
  </code></pre>