db={"posts":[{post:{subject:"test",category:123,author:1}}],"users":[{id:1,name:"foo"}],"category":[{id:123,name:"bar"}]}

db.posts.aggregate([{"$lookup":{"from":"users","localField":"post.author","foreignField":"id","as":"userJoin"}},{"$lookup":{"from":"category","localField":"post.category","foreignField":"id","as":"categoryJoin"}},{$match:{"categoryJoin.name":"bar","userJoin.name":"foo"}}])