[{_id:"Vanilla Sundae",keywords:["vanilla","ice cream","desert"]},{_id:"Vanilla Cake",keywords:["vanilla","cake","baking","desert"]},{_id:"Chocolate Cake",keywords:["chocolate","cake","baking","desert"]}]

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