 <p>Problem since solved. I was given the answer by Aaron Heckmann over on the <a href="https://groups.google.com/forum/#!topic/mongoose-orm/0p-rhtcJdng">mongoose Google Group</a>:</p> <blockquote> <p>Always declare your child schemas before passing them to you parent schemas otherwise you are passing undefined. </p> <p>SubCommentSchema should be first, then Comment followed by BlogPost.</p> </blockquote> <p>After reversing the schemas it worked.</p>