[{"id":"1","model":"thunderbolt","construction":"bolt-on","scale":19,"wood":{"body":"oak","top":"maple","fretboard":"rosewood"}},{"id":"2","model":"model2","construction":"const2","scale":19,"wood":{"body":"oak2","top":"maple","fretboard":"rosewood"}},{"id":"3","model":"thunderbolt","construction":"const2","scale":19,"wood":{"body":"oak","top":"maple","fretboard":"rosewood"}}]

db.collection.aggregate([{"$unwind":"$wood"},{"$group":{"_id":"$wood.body","count":{"$sum":1}}}])