聚合框架概览:
db.reviews.aggregate([{$group:{_id:'$product_id',count:{$sum:1}}}]);
product = db.products.findOne({'slug':'wheelbarrow-9092'});
ratingSummary = db.review.aggregate([
{$match:{'product_id':product['_id']}},
{$group:{_id:'$product_id',
average:{$avg:'$rating'},
count:{$sum:1}}}
])
$project
$group:
$match、$sort、$skip、$limit
$out