title: Abort due to constraint violation columns GroupId, idx are not unique
body: <p>I'm using FluentNHibernate and have done a many-to-many mapping but when I try to save my entity I get the following error:</p>
  
  <pre><code>NHibernate.Exceptions.GenericADOException: NHibernate.Exceptions.GenericADOException
  : could not insert collection: [Test.Entities.Recipient.Groups#b6815d34-f436-4142-9b8e-1bfcbf25509e][SQL: SQL not available]
  ---- System.Data.SQLite.SQLiteException : Abort due to constraint violation
  columns GroupId, idx are not unique
  </code></pre>
  
  <p>Here is my mapping:</p>
  
  <pre><code>public class GroupMap : ClassMap&lt;Group&gt;
  {
      public GroupMap()
      {
          Id(x =&gt; x.Id).GeneratedBy.Guid();
          Map(x =&gt; x.Name);
          Map(x =&gt; x.SenderName);
          Map(x =&gt; x.Created);
          HasManyToMany(x =&gt; x.Recipients)
              .AsList()
              .WithTableName("groups_recipients")
              .WithParentKeyColumn("GroupId")
              .WithChildKeyColumn("RecipientId")
              .LazyLoad()
              .Cascade.AllDeleteOrphan();
      }
  }
  
  public class RecipientMap : ClassMap&lt;Recipient&gt;
  {
      public RecipientMap()
      {
          Id(x =&gt; x.Id).GeneratedBy.Guid();
          Map(x =&gt; x.Firstname);
          Map(x =&gt; x.Lastname);
          Map(x =&gt; x.Phone);
          Map(x =&gt; x.Email);
          HasManyToMany(x =&gt; x.Groups)
              .AsList()
              .WithTableName("groups_recipients")
              .WithParentKeyColumn("RecipientId")
              .WithChildKeyColumn("GroupId")
              .LazyLoad().Cascade.None();
      }
  }
  </code></pre>
  
  <p>The problem seems to have something to do with the relationship tables id but I can't figure out how to solve it.</p>
  
  <p>Cheers,
  nandarya</p>