Database
db={"news":[{id:"123123123",category:[1,2]},{id:"123123123",category:[]},{id:"123123123",category:[2]}],"category":[{name:"123a",_id:1,title:"blah blah"},{name:"987a",_id:2,title:"blih blah"}]}
Query
db.category.aggregate([{$lookup:{from:"news",localField:"_id",foreignField:"category",as:"count"}},{$set:{count:{$size:"$count"}}}])