title: EF Code First composite key mapping
body: <p>I'm using the CTP 5 of EF 4 and Code first.
  I don't get a many-many relation working with a composite key on one side.</p>
  
  <pre><code>    modelBuilder.Entity&lt;Item&gt;()                 
             .HasMany(i =&gt; i.Categories)  
             .WithMany(o =&gt; o.Items)  
             .Map(
             mc =&gt;
             {                  
                 mc.ToTable("ItemCategories");  
                 mc.MapLeftKey(i =&gt; i.Id, "ItemId");  
                 mc.MapRightKey(o =&gt; o.TemplateID, "TemplateId");  
                 mc.MapRightKey(o =&gt; o.ItemId, "ItemId");                  
             }
             );
  </code></pre>
  
  <p>So instead of having a simple key for Categories in my matching table, I've got a composite one. And one part of the composite key is also the key for the Item type,
  which seems to be the problem here. </p>
  
  <p>I get the error:  "Each property name in a type must be unique. Property name 'ItemId' was already defined."</p>
  
  <p>How can I configure EF to use a composite key in this case?</p>