MongoDB中不同collections的聚合

Question

不知道该怎么做，有 SQL 的背景，在那里实现我需要做的事情要容易得多。

基本上我有这样的收藏新闻：

const news = [
{id: '123123123', category: [ObjectID(12312312323), ObjectID(987987987)]},
{id: '123123123', category: []},
{id: '123123123', category: [ObjectID(987987987)]}
]

然后我有这样的集合类别：

const category = [
{name: "123a", _id: ObjectID(123123123}, title:"blah blah" },
{name: "987a", _id: ObjectID(987987987}, title:"blih blah" ,
]

我需要做的是加入两个表并获得像这样的聚合

[
{name: "987a", title: "blih blah", count: 2},
{name: "123a", title: "blah blah", count: 1}
 
]

所以即使在 MySQL 我也会得到类似的东西：

SELECT name, title, count(Select count(category) from news) from categories 

但我没有用 mongoDb / mongoose 得到它。

Answer 1

• $lookup加入news集合，传递_id作为 localField 和category作为 foreignField
• $size获取新集合结果的总元素count

db.category.aggregate([
  {
    $lookup: {
      from: "news",
      localField: "_id",
      foreignField: "category",
      as: "count"
    }
  },
  { $set: { count: { $size: "$count" } } }
])

操场